[AZ-393] [AZ-394] [AZ-395] C8 outbound chain + AP MAVLink2 signing

AZ-393 ArduPilot outbound: PymavlinkArdupilotAdapter encodes
EstimatorOutput to MAVLink2 GPS_INPUT via gps_input_send; emits
NAMED_VALUE_FLOAT(name="src_lbl") every frame and STATUSTEXT on
source_label transition (1 Hz per-severity cap). Smoothed-output
guard (Invariant 6), single-writer thread (Invariant 8), SPD
propagation. Shared helper _outbound_provenance.py owns the
canonical source-label-to-float table + transition rate-limiter.

AZ-394 iNav outbound: Msp2InavAdapter encodes EstimatorOutput to
hand-rolled MSP2_SENSOR_GPS (0x1F03, 52-byte LE payload via
_msp2_sensor_gps_encoder.py + YAMSPy send_RAW_msg). Secondary
unsigned MAVLink channel for STATUSTEXT transitions. open()
rejects non-None signing_key (RESTRICT-COMM-2 / Invariant 2);
request_source_set_switch raises SourceSetSwitchNotSupportedError
(Invariant 9 verified: never calls setup_signing on secondary).

AZ-395 AP MAVLink2 signing: ephemeral per-flight 32-byte key
from secrets.token_bytes; pymavlink setup_signing handshake at
open(); in-place bytearray zeroisation on close(); mid-flight
signing-failure detection (ERROR log + WARNING STATUSTEXT + no
raise; threshold configurable). Key never logged / persisted /
serialised (regex-scanned by AC-4/AC-5). BUILD_DEV_STATIC_KEY=ON
enables repeatable static-key dev path; rejected at open() when
the build flag is absent.

Shared: EstimatorOutput.smoothed (default False) added for the
Invariant 6 gate at the C8 boundary; FcConfig extended with
dev_static_signing_key + signing_failure_threshold (additive
defaults; cross-field validation in __post_init__).

Tests: 33 new AC tests (11 + 11 + 11) covering all 30 ACs; full
suite 476 passing / 2 skipped / 0 failing (was 443). Contract
surfaces unchanged at fc_adapter_protocol v1.0.0 and
composition_root v1.2.0.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/gps_denied_onboard/components/c8_fc_adapter/_msp2_sensor_gps_encoder.py

@@ -0,0 +1,156 @@
+"""MSP2_SENSOR_GPS wire-format encoder/decoder (AZ-394 / E-C8).
+
+iNav-specific message ``MSP2_SENSOR_GPS = 0x1F03`` — host-to-FC
+external GPS injection. YAMSPy's MSP code table is BetaFlight-derived
+and does NOT include this iNav-only code, so we encode the payload by
+hand and ship it through the transport's ``send_RAW_msg(code, data)``
+entry point.
+
+Wire payload (little-endian, packed; iNav source of truth:
+``inav/src/main/io/msp.c`` — search ``MSP2_SENSOR_GPS``):
+
+| Offset | Field                 | Type   | Notes                              |
+|--------|-----------------------|--------|------------------------------------|
+| 0      | instance              | u8     | 0 (single external GPS instance)   |
+| 1      | gps_week              | u16    | 0 (unused for external injection)  |
+| 3      | ms_tow                | u32    | 0 (unused)                         |
+| 7      | fix_type              | u8     | 3 (3D fix); contract Invariant 5   |
+| 8      | satellites_visible    | u8     | cosmetic; we send 10                |
+| 9      | h_pos_accuracy_mm     | u16    | from CovarianceProjector (clamped) |
+| 11     | v_pos_accuracy_mm     | u16    | 0 (we don't project vertical)      |
+| 13     | h_vel_accuracy_mm_s   | u16    | 0                                  |
+| 15     | hdop                  | u16    | 0                                  |
+| 17     | longitude_e7          | i32    | wgs84 lon * 1e7                    |
+| 21     | latitude_e7           | i32    | wgs84 lat * 1e7                    |
+| 25     | msl_altitude_cm       | i32    | wgs84 alt * 100                    |
+| 29     | ned_vel_north_cm_s    | i32    | 0                                  |
+| 33     | ned_vel_east_cm_s     | i32    | 0                                  |
+| 37     | ned_vel_down_cm_s     | i32    | 0                                  |
+| 41     | ground_course_cdeg    | u16    | 0                                  |
+| 43     | true_yaw_cdeg         | u16    | 0                                  |
+| 45     | year                  | u16    | 0                                  |
+| 47     | month                 | u8     | 0                                  |
+| 48     | day                   | u8     | 0                                  |
+| 49     | hour                  | u8     | 0                                  |
+| 50     | min                   | u8     | 0                                  |
+| 51     | sec                   | u8     | 0                                  |
+| 52     | (end)                 |        | total length = 52 bytes            |
+"""
+
+from __future__ import annotations
+
+import struct
+from dataclasses import dataclass
+from typing import Final
+
+__all__ = [
+    "MSP2_SENSOR_GPS_CODE",
+    "MSP2_SENSOR_GPS_PAYLOAD_LEN",
+    "Msp2SensorGpsPayload",
+    "decode_msp2_sensor_gps",
+    "encode_msp2_sensor_gps",
+]
+
+
+MSP2_SENSOR_GPS_CODE: Final[int] = 0x1F03
+MSP2_SENSOR_GPS_PAYLOAD_LEN: Final[int] = 52
+
+# little-endian, no padding; matches inav's __packed__ struct layout.
+_PACK_FMT: Final[str] = "<BHIBBHHHHiiiiiiHHHBBBBB"
+
+
+@dataclass(frozen=True, slots=True)
+class Msp2SensorGpsPayload:
+    """Decoded MSP2_SENSOR_GPS payload (subset we exercise)."""
+
+    instance: int
+    fix_type: int
+    satellites_visible: int
+    h_pos_accuracy_mm: int
+    longitude_e7: int
+    latitude_e7: int
+    msl_altitude_cm: int
+
+
+def encode_msp2_sensor_gps(
+    *,
+    fix_type: int,
+    satellites_visible: int,
+    h_pos_accuracy_mm: int,
+    longitude_e7: int,
+    latitude_e7: int,
+    msl_altitude_cm: int,
+    ned_vel_north_cm_s: int = 0,
+    ned_vel_east_cm_s: int = 0,
+    ned_vel_down_cm_s: int = 0,
+) -> bytes:
+    """Encode an MSP2_SENSOR_GPS payload (52 bytes)."""
+    return struct.pack(
+        _PACK_FMT,
+        0,  # instance
+        0,  # gps_week
+        0,  # ms_tow
+        int(fix_type),
+        int(satellites_visible),
+        int(h_pos_accuracy_mm) & 0xFFFF,
+        0,  # v_pos_accuracy_mm
+        0,  # h_vel_accuracy_mm_s
+        0,  # hdop
+        int(longitude_e7),
+        int(latitude_e7),
+        int(msl_altitude_cm),
+        int(ned_vel_north_cm_s),
+        int(ned_vel_east_cm_s),
+        int(ned_vel_down_cm_s),
+        0,  # ground_course_cdeg
+        0,  # true_yaw_cdeg
+        0,  # year
+        0,
+        0,
+        0,
+        0,
+        0,  # month/day/hour/min/sec
+    )
+
+
+def decode_msp2_sensor_gps(buf: bytes) -> Msp2SensorGpsPayload:
+    """Decode the subset of MSP2_SENSOR_GPS we exercise."""
+    if len(buf) != MSP2_SENSOR_GPS_PAYLOAD_LEN:
+        raise ValueError(
+            f"MSP2_SENSOR_GPS expects {MSP2_SENSOR_GPS_PAYLOAD_LEN} bytes; got {len(buf)}"
+        )
+    unpacked = struct.unpack(_PACK_FMT, buf)
+    (
+        instance,
+        _gps_week,
+        _ms_tow,
+        fix_type,
+        satellites_visible,
+        h_pos_accuracy_mm,
+        _v_pos_acc,
+        _h_vel_acc,
+        _hdop,
+        longitude_e7,
+        latitude_e7,
+        msl_altitude_cm,
+        _vn,
+        _ve,
+        _vd,
+        _gc,
+        _yaw,
+        _year,
+        _mo,
+        _d,
+        _h,
+        _mn,
+        _s,
+    ) = unpacked
+    return Msp2SensorGpsPayload(
+        instance=instance,
+        fix_type=fix_type,
+        satellites_visible=satellites_visible,
+        h_pos_accuracy_mm=h_pos_accuracy_mm,
+        longitude_e7=longitude_e7,
+        latitude_e7=latitude_e7,
+        msl_altitude_cm=msl_altitude_cm,
+    )

src/gps_denied_onboard/components/c8_fc_adapter/_outbound_provenance.py

@@ -0,0 +1,107 @@
+"""Outbound provenance side-channel helpers (AZ-393 / AZ-394 / E-C8).
+
+Two pieces shared by AP and iNav outbound paths:
+
+1. :func:`source_label_to_float` — deterministic ``source_label`` →
+   ``float`` mapping consumed by AP's ``NAMED_VALUE_FLOAT(name="src_lbl")``
+   side-channel. The OPERATOR-side decoder (E-C12) MUST use the SAME
+   mapping; the canonical table lives here.
+
+2. :class:`StatusTextTransitionRateLimiter` — emits ``STATUSTEXT(...)``
+   exactly once per ``source_label`` transition (AC-4 / AZ-393 AC-3 /
+   AZ-394 AC-3), with a defensive 1-per-second per-severity hard cap
+   against pathological transition spam (constraint § 4 of AZ-393).
+
+The rate limiter has no I/O; the adapter passes ``send_statustext``
+in. This keeps the helper pure for unit testing and matches the
+per-adapter wire dispatch responsibility.
+"""
+
+from __future__ import annotations
+
+import threading
+import time
+from collections.abc import Callable
+from typing import Final
+
+from gps_denied_onboard._types.fc import Severity
+
+__all__ = [
+    "SOURCE_LABEL_TO_FLOAT",
+    "StatusTextTransitionRateLimiter",
+    "source_label_to_float",
+]
+
+
+# Canonical source-label-to-float mapping (AZ-393 AC-3 / D-C8-7).
+# Operator-side decoder in C12 MUST mirror this table.
+SOURCE_LABEL_TO_FLOAT: Final[dict[str, float]] = {
+    "unknown": 0.0,
+    "visual_propagated": 1.0,
+    "sat_anchored": 2.0,
+    "imu_only": 3.0,
+    "warm_start": 4.0,
+    "smoothed": 5.0,
+    "ac52_fallback": 6.0,
+}
+
+
+def source_label_to_float(label: str) -> float:
+    """Return the canonical float encoding for ``label``; unknowns map to 0.0."""
+    return SOURCE_LABEL_TO_FLOAT.get(label, SOURCE_LABEL_TO_FLOAT["unknown"])
+
+
+class StatusTextTransitionRateLimiter:
+    """Emits a STATUSTEXT exactly once per ``source_label`` transition.
+
+    Defensive secondary cap: at most one emit per second per severity,
+    in case a producer flaps source_label every frame at 5 Hz.
+
+    Thread-safe; ``note_label_and_maybe_emit`` may be called from any
+    thread. The send callback is invoked OUTSIDE the lock.
+    """
+
+    def __init__(
+        self,
+        send_statustext: Callable[[str, Severity], None],
+        *,
+        min_interval_s: float = 1.0,
+        clock: Callable[[], float] = time.monotonic,
+    ) -> None:
+        self._send = send_statustext
+        self._min_interval_s = min_interval_s
+        self._clock = clock
+        self._lock = threading.Lock()
+        self._last_label: str | None = None
+        self._last_emit_at_by_sev: dict[Severity, float] = {}
+
+    def note_label_and_maybe_emit(
+        self,
+        new_label: str,
+        *,
+        severity: Severity = Severity.INFO,
+    ) -> bool:
+        """Record ``new_label`` and emit a STATUSTEXT iff this is a transition.
+
+        Returns ``True`` if a STATUSTEXT was emitted, ``False`` otherwise.
+        """
+        with self._lock:
+            if self._last_label == new_label:
+                return False
+            previous = self._last_label
+            self._last_label = new_label
+            # Hard cap: at most one emit per second per severity.
+            now = self._clock()
+            last_emit = self._last_emit_at_by_sev.get(severity, float("-inf"))
+            if (now - last_emit) < self._min_interval_s:
+                return False
+            self._last_emit_at_by_sev[severity] = now
+            msg = f"src={new_label}" if previous is None else f"src {previous}->{new_label}"
+        # Send OUTSIDE the lock — pymavlink statustext_send may block on UART.
+        self._send(msg, severity)
+        return True
+
+    @property
+    def last_label(self) -> str | None:
+        with self._lock:
+            return self._last_label

src/gps_denied_onboard/components/c8_fc_adapter/msp2_inav_adapter.py

@@ -0,0 +1,283 @@
+"""iNav MSP2 + secondary-MAVLink adapter (AZ-394 / E-C8).
+
+Outbound: encodes ``EstimatorOutput`` to MSP2_SENSOR_GPS and writes
+through a YAMSPy-style transport (``send_RAW_msg(code, data)``).
+Side-channel: STATUSTEXT on the SECONDARY MAVLink telemetry link
+(NOT on the primary MSP2 link) — iNav supports MAVLink for telemetry
+output but not for primary positioning per RESTRICT-COMM-2. The
+secondary link is ALWAYS unsigned (Invariant 9).
+
+Build flag: ``BUILD_FC_INAV``.
+"""
+
+from __future__ import annotations
+
+import threading
+import time
+from collections.abc import Callable
+from typing import Any, Final
+
+from gps_denied_onboard._types.emitted import EmittedExternalPosition
+from gps_denied_onboard._types.fc import (
+    FcKind,
+    FlightState,
+    FlightStateSignal,
+    PortConfig,
+    Severity,
+    Subscription,
+    TelemetryCallback,
+)
+from gps_denied_onboard._types.geo import LatLonAlt
+from gps_denied_onboard._types.pose import EstimatorOutput
+from gps_denied_onboard.components.c8_fc_adapter._covariance_projector import (
+    CovarianceProjector,
+)
+from gps_denied_onboard.components.c8_fc_adapter._msp2_sensor_gps_encoder import (
+    MSP2_SENSOR_GPS_CODE,
+    encode_msp2_sensor_gps,
+)
+from gps_denied_onboard.components.c8_fc_adapter._outbound_provenance import (
+    StatusTextTransitionRateLimiter,
+)
+from gps_denied_onboard.components.c8_fc_adapter._subscription import SubscriptionBus
+from gps_denied_onboard.components.c8_fc_adapter.errors import (
+    FcAdapterConfigError,
+    FcEmitError,
+    FcOpenError,
+    SourceSetSwitchNotSupportedError,
+)
+from gps_denied_onboard.config import Config
+from gps_denied_onboard.fdr_client.client import FdrClient
+from gps_denied_onboard.logging import get_logger
+
+__all__ = ["Msp2InavAdapter"]
+
+
+_FIX_TYPE_3D: Final[int] = 3
+_INAV_DEFAULT_SATS: Final[int] = 10
+
+
+def _mav_severity(sev: Severity) -> int:
+    return int(sev.value)
+
+
+class Msp2InavAdapter:
+    """iNav FcAdapter (MSP2 primary + unsigned MAVLink secondary)."""
+
+    def __init__(
+        self,
+        *,
+        config: Config,
+        wgs_converter: Any,
+        covariance_projector: CovarianceProjector,
+        fdr_client: FdrClient,
+        clock: Callable[[], float] = time.monotonic,
+        msp_connect_factory: Callable[[str, int], Any] | None = None,
+        secondary_mavlink_factory: Callable[[], Any] | None = None,
+    ) -> None:
+        self._config = config
+        self._wgs_converter = wgs_converter
+        self._cov_projector = covariance_projector
+        self._fdr_client = fdr_client
+        self._clock = clock
+        self._msp_connect_factory = msp_connect_factory
+        self._secondary_mavlink_factory = secondary_mavlink_factory
+        self._log = get_logger("c8_fc_adapter.inav_adapter")
+        # Wire state ------------------------------------------------------
+        self._msp: Any = None
+        self._secondary_mav: Any = None
+        self._opened = False
+        self._sequence_number = 0
+        self._first_emit_logged = False
+        self._open_emit_thread_ident: int | None = None
+        # Inbound bus stub — AZ-394 doesn't wire the inbound (iNav
+        # polling decoder lands in AZ-391; the per-adapter inbound
+        # composition happens in a follow-up batch).
+        self._bus = SubscriptionBus()
+        # Provenance rate-limiter for the secondary MAVLink STATUSTEXT.
+        self._provenance = StatusTextTransitionRateLimiter(
+            send_statustext=self._send_statustext_secondary,
+            clock=time.monotonic,
+        )
+
+    # ------------------------------------------------------------------
+    # FcAdapter Protocol implementation
+
+    def open(self, port: PortConfig, signing_key: bytes | None) -> None:
+        if self._opened:
+            raise FcOpenError("Msp2InavAdapter already opened")
+        if port.fc_kind is not FcKind.INAV:
+            raise FcOpenError(f"Msp2InavAdapter requires FcKind.INAV; got {port.fc_kind!r}")
+        if signing_key is not None:
+            # Invariant 2 / RESTRICT-COMM-2 — iNav has no signing.
+            raise FcAdapterConfigError("iNav does not support MAVLink signing per RESTRICT-COMM-2")
+        try:
+            self._msp = self._connect_msp(port)
+        except Exception as exc:
+            raise FcOpenError(f"MSP2 connect failed: {exc!r}") from exc
+        try:
+            self._secondary_mav = (
+                self._secondary_mavlink_factory()
+                if self._secondary_mavlink_factory is not None
+                else None
+            )
+        except Exception as exc:
+            self._log.warning(
+                f"c8.inav.secondary_mavlink_open_failed: {exc!r}",
+                extra={
+                    "kind": "c8.inav.secondary_mavlink_open_failed",
+                    "kv": {"error": repr(exc)},
+                },
+            )
+            self._secondary_mav = None
+        self._opened = True
+
+    def close(self) -> None:
+        if not self._opened:
+            return
+        try:
+            for conn in (self._msp, self._secondary_mav):
+                if conn is not None and hasattr(conn, "close"):
+                    try:
+                        conn.close()
+                    except Exception:
+                        pass
+        finally:
+            self._opened = False
+            self._msp = None
+            self._secondary_mav = None
+            self._open_emit_thread_ident = None
+            self._first_emit_logged = False
+
+    def subscribe_telemetry(self, callback: TelemetryCallback) -> Subscription:
+        return self._bus.subscribe(callback)
+
+    def emit_external_position(self, output: EstimatorOutput) -> EmittedExternalPosition:
+        if not self._opened or self._msp is None:
+            raise FcEmitError("adapter not opened")
+        self._enforce_single_writer()
+        if output.smoothed:
+            self._log_emit_failed("smoothed_rejected", output.frame_id)
+            raise FcEmitError("smoothed output cannot be emitted to FC (Invariant 6)")
+        h_pos_accuracy_mm = self._cov_projector.to_inav_h_pos_accuracy_mm(output)
+        wgs = self._extract_wgs84(output)
+        emitted_at = time.monotonic_ns()
+        self._sequence_number = (self._sequence_number + 1) & 0xFF
+        seq = self._sequence_number
+        payload = encode_msp2_sensor_gps(
+            fix_type=_FIX_TYPE_3D,
+            satellites_visible=_INAV_DEFAULT_SATS,
+            h_pos_accuracy_mm=h_pos_accuracy_mm,
+            longitude_e7=int(wgs.lon_deg * 1e7),
+            latitude_e7=int(wgs.lat_deg * 1e7),
+            msl_altitude_cm=int(wgs.alt_m * 100.0),
+        )
+        try:
+            self._msp.send_RAW_msg(MSP2_SENSOR_GPS_CODE, payload)
+        except Exception as exc:
+            self._log_emit_failed(repr(exc), output.frame_id)
+            raise FcEmitError(f"iNav MSP2 outbound wire emit failed: {exc!r}") from exc
+        self._provenance.note_label_and_maybe_emit(output.source_label, severity=Severity.INFO)
+        if not self._first_emit_logged:
+            self._first_emit_logged = True
+            self._log.info(
+                "c8.inav.first_emit",
+                extra={
+                    "kind": "c8.inav.first_emit",
+                    "kv": {"frame_id": output.frame_id, "seq": seq},
+                },
+            )
+        self._log.debug(
+            "c8.inav.emit",
+            extra={
+                "kind": "c8.inav.emit",
+                "kv": {
+                    "frame_seq": seq,
+                    "h_pos_accuracy_mm": h_pos_accuracy_mm,
+                    "source_label": output.source_label,
+                },
+            },
+        )
+        return EmittedExternalPosition(
+            fc_kind=FcKind.INAV,
+            horiz_accuracy_m=h_pos_accuracy_mm / 1000.0,
+            source_label=output.source_label,
+            emitted_at=emitted_at,
+            sequence_number=seq,
+        )
+
+    def emit_status_text(self, msg: str, severity: Severity) -> None:
+        if not self._opened:
+            raise FcEmitError("adapter not opened")
+        self._enforce_single_writer()
+        self._send_statustext_secondary(msg, severity)
+
+    def request_source_set_switch(self) -> None:
+        raise SourceSetSwitchNotSupportedError("iNav: no MAV_CMD_SET_EKF_SOURCE_SET equivalent")
+
+    def current_flight_state(self) -> FlightStateSignal:
+        # Inbound iNav state ring is not wired into this adapter shell;
+        # the inbound decoder (AZ-391) exposes its own state_ring that
+        # the composition root can read directly. Default to INIT here.
+        return FlightStateSignal(
+            state=FlightState.INIT,
+            last_valid_gps_hint_wgs84=None,
+            last_valid_gps_age_ms=None,
+            captured_at=time.monotonic_ns(),
+        )
+
+    # ------------------------------------------------------------------
+    # Internals
+
+    def _enforce_single_writer(self) -> None:
+        cur = threading.get_ident()
+        if self._open_emit_thread_ident is None:
+            self._open_emit_thread_ident = cur
+            return
+        if self._open_emit_thread_ident != cur:
+            raise RuntimeError(
+                "Msp2InavAdapter outbound is single-writer; "
+                f"first thread={self._open_emit_thread_ident}, this thread={cur}"
+            )
+
+    def _connect_msp(self, port: PortConfig) -> Any:
+        if self._msp_connect_factory is not None:
+            return self._msp_connect_factory(port.device, port.baud)
+        # Lazy import so the test path with an injected factory does NOT
+        # require yamspy at module-import time.
+        from yamspy import MSPy
+
+        return MSPy(device=port.device, baudrate=port.baud)
+
+    def _send_statustext_secondary(self, msg: str, severity: Severity) -> None:
+        if self._secondary_mav is None:
+            return
+        text = msg.encode("utf-8")[:50]
+        try:
+            self._secondary_mav.mav.statustext_send(_mav_severity(severity), text)
+        except Exception as exc:
+            self._log.debug(
+                f"c8.inav.secondary_statustext_failed: {exc!r}",
+                extra={
+                    "kind": "c8.inav.secondary_statustext_failed",
+                    "kv": {"error": repr(exc)},
+                },
+            )
+
+    def _extract_wgs84(self, output: EstimatorOutput) -> LatLonAlt:
+        wgs = output.extras.get("wgs84") if output.extras else None
+        if not isinstance(wgs, LatLonAlt):
+            raise FcEmitError(
+                "EstimatorOutput.extras['wgs84'] missing or not a LatLonAlt; "
+                "composition root must inject the ENU->WGS84 enricher"
+            )
+        return wgs
+
+    def _log_emit_failed(self, reason: str, frame_id: int) -> None:
+        self._log.error(
+            f"c8.inav.emit_failed: {reason}",
+            extra={
+                "kind": "c8.inav.emit_failed",
+                "kv": {"reason": reason, "frame_id": frame_id},
+            },
+        )

src/gps_denied_onboard/components/c8_fc_adapter/pymavlink_ardupilot_adapter.py

@@ -0,0 +1,499 @@
+"""ArduPilot MAVLink 2.0 adapter (AZ-393 outbound, AZ-395 signing, E-C8).
+
+Composes :class:`PymavlinkInboundDecoder` (AZ-391) for the inbound
+producer side and the local outbound encoder for `emit_external_position` +
+`emit_status_text`. Implements `FcAdapter` per the contract.
+
+Production wiring (composition-root, AZ-393 / AZ-395 follow-up):
+- Constructor receives the WgsConverter (AZ-279), CovarianceProjector
+  (AZ-392), FdrClient (AZ-273), and a clock callable.
+- `open(port, signing_key)` opens the pymavlink connection on the
+  configured port and, if signing is enabled in config, completes
+  the per-flight signing handshake (AZ-395).
+- A single composition-root thread is bound for outbound emit via
+  :func:`bind_outbound_emit_thread`; emitting from a second thread
+  raises :class:`RuntimeError`.
+
+Build flag: ``BUILD_FC_ARDUPILOT_PLANE``.
+"""
+
+from __future__ import annotations
+
+import os
+import secrets
+import threading
+import time
+from collections.abc import Callable
+from datetime import datetime, timezone
+from typing import Any, Final
+
+from gps_denied_onboard._types.emitted import EmittedExternalPosition
+from gps_denied_onboard._types.fc import (
+    FcKind,
+    FlightState,
+    FlightStateSignal,
+    PortConfig,
+    Severity,
+    Subscription,
+    TelemetryCallback,
+)
+from gps_denied_onboard._types.geo import LatLonAlt
+from gps_denied_onboard._types.pose import EstimatorOutput
+from gps_denied_onboard.components.c8_fc_adapter._covariance_projector import (
+    CovarianceProjector,
+)
+from gps_denied_onboard.components.c8_fc_adapter._inbound_mavlink import (
+    PymavlinkInboundDecoder,
+)
+from gps_denied_onboard.components.c8_fc_adapter._outbound_provenance import (
+    StatusTextTransitionRateLimiter,
+    source_label_to_float,
+)
+from gps_denied_onboard.components.c8_fc_adapter._subscription import SubscriptionBus
+from gps_denied_onboard.components.c8_fc_adapter.errors import (
+    FcEmitError,
+    FcOpenError,
+    SigningHandshakeError,
+)
+from gps_denied_onboard.config import Config
+from gps_denied_onboard.fdr_client.client import FdrClient
+from gps_denied_onboard.fdr_client.records import FdrRecord
+from gps_denied_onboard.logging import get_logger
+
+__all__ = ["PymavlinkArdupilotAdapter"]
+
+
+_GPS_FIX_TYPE_3D: Final[int] = 3
+_NAMED_VALUE_FLOAT_NAME: Final[str] = "src_lbl"
+_SIGNING_KEY_LEN: Final[int] = 32
+_BUILD_DEV_STATIC_KEY_ENV: Final[str] = "BUILD_DEV_STATIC_KEY"
+
+
+# Maps Severity enum to MAVLink statustext severity numeric value.
+def _mav_severity(sev: Severity) -> int:
+    return int(sev.value)
+
+
+class PymavlinkArdupilotAdapter:
+    """ArduPilot Plane FcAdapter (MAVLink 2.0).
+
+    Threading: outbound (``emit_*``) is bound to a single composition-
+    root thread; inbound (decode loop) runs on its own thread inside
+    ``PymavlinkInboundDecoder.run_decode_loop`` (AZ-391).
+    """
+
+    def __init__(
+        self,
+        *,
+        config: Config,
+        wgs_converter: Any,
+        covariance_projector: CovarianceProjector,
+        fdr_client: FdrClient,
+        clock: Callable[[], float] = time.monotonic,
+        flight_id: str = "",
+        connect_factory: Callable[[str, int], Any] | None = None,
+    ) -> None:
+        self._config = config
+        self._wgs_converter = wgs_converter
+        self._cov_projector = covariance_projector
+        self._fdr_client = fdr_client
+        self._clock = clock
+        self._flight_id = flight_id
+        self._connect_factory = connect_factory
+        self._signing_failure_threshold = max(1, int(config.fc.signing_failure_threshold))
+        self._log = get_logger("c8_fc_adapter.ap_adapter")
+        # Wire state ------------------------------------------------------
+        self._connection: Any = None
+        self._signing_key: bytearray | None = None
+        self._opened = False
+        self._sequence_number = 0
+        self._first_emit_logged = False
+        self._open_emit_thread_ident: int | None = None
+        self._signing_failure_logged_at_count = 0
+        # Inbound bus + decoder (lazily constructed inside ``open``).
+        self._bus = SubscriptionBus()
+        self._inbound: PymavlinkInboundDecoder | None = None
+        self._inbound_thread: threading.Thread | None = None
+        # Outbound provenance rate limiter.
+        self._provenance = StatusTextTransitionRateLimiter(
+            send_statustext=self._send_statustext_internal,
+            clock=time.monotonic,
+        )
+
+    # ------------------------------------------------------------------
+    # FcAdapter Protocol implementation
+
+    def open(self, port: PortConfig, signing_key: bytes | None) -> None:
+        if self._opened:
+            raise FcOpenError("PymavlinkArdupilotAdapter already opened")
+        if port.fc_kind is not FcKind.ARDUPILOT_PLANE:
+            raise FcOpenError(
+                f"PymavlinkArdupilotAdapter requires FcKind.ARDUPILOT_PLANE; got {port.fc_kind!r}"
+            )
+        try:
+            self._connection = self._connect(port)
+        except Exception as exc:
+            raise FcOpenError(f"pymavlink connect failed: {exc!r}") from exc
+        # Per-flight signing key handling (AZ-395). ----------------------
+        # In production, `signing_key_source = "ephemeral_per_flight"`; we
+        # generate a fresh 32-byte key. The caller (composition root) is
+        # also free to pass an explicit signing_key for test paths.
+        source = self._config.fc.signing_key_source
+        if source == "none":
+            pass
+        elif source == "ephemeral_per_flight":
+            self._signing_key = self._materialise_signing_key(signing_key)
+            try:
+                self._setup_signing(self._signing_key)
+            except Exception as exc:
+                self._handle_signing_handshake_failure(exc)
+                raise SigningHandshakeError(f"AP signing handshake failed: {exc!r}") from exc
+            self._fdr_signing_event(
+                kind="c8.ap.signing_key_rotated",
+                kv={"flight_id": self._flight_id, "key_age_s": 0},
+            )
+        elif source == "dev_static":
+            if os.environ.get(_BUILD_DEV_STATIC_KEY_ENV) != "ON":
+                raise FcOpenError(
+                    "signing_key_source='dev_static' requires "
+                    f"{_BUILD_DEV_STATIC_KEY_ENV}=ON at build time"
+                )
+            self._signing_key = self._materialise_dev_static_key()
+            try:
+                self._setup_signing(self._signing_key)
+            except Exception as exc:
+                self._handle_signing_handshake_failure(exc)
+                raise SigningHandshakeError(f"AP signing handshake failed: {exc!r}") from exc
+            self._log.warning(
+                "c8.ap.signing_dev_static_key",
+                extra={"kind": "c8.ap.signing_dev_static_key", "kv": {}},
+            )
+        else:
+            raise FcOpenError(f"unknown signing_key_source={source!r}")
+        # Inbound decoder + thread.
+        self._inbound = PymavlinkInboundDecoder(self._connection, self._bus)
+        thread = threading.Thread(
+            target=self._inbound.run_decode_loop,
+            name="c8.ap.inbound",
+            daemon=True,
+        )
+        self._inbound_thread = thread
+        thread.start()
+        self._open_emit_thread_ident = None
+        self._opened = True
+
+    def close(self) -> None:
+        if not self._opened:
+            return
+        if self._inbound is not None:
+            self._inbound.stop()
+        # Zeroise the signing-key buffer BEFORE we let it be deallocated
+        # (AZ-395 AC-7 / Invariant 10).
+        if self._signing_key is not None:
+            self._zeroise_signing_key(self._signing_key)
+            self._log.info(
+                "c8.ap.signing_key_zeroised",
+                extra={"kind": "c8.ap.signing_key_zeroised", "kv": {}},
+            )
+            self._signing_key = None
+        try:
+            if self._connection is not None and hasattr(self._connection, "close"):
+                self._connection.close()
+        finally:
+            self._opened = False
+            self._connection = None
+            self._inbound = None
+            self._open_emit_thread_ident = None
+            self._first_emit_logged = False
+
+    def subscribe_telemetry(self, callback: TelemetryCallback) -> Subscription:
+        return self._bus.subscribe(callback)
+
+    def emit_external_position(self, output: EstimatorOutput) -> EmittedExternalPosition:
+        if not self._opened or self._connection is None:
+            raise FcEmitError("adapter not opened")
+        self._enforce_single_writer()
+        if output.smoothed:
+            self._log_emit_failed("smoothed_rejected", output.frame_id)
+            raise FcEmitError("smoothed output cannot be emitted to FC (Invariant 6)")
+        horiz_accuracy_m = self._cov_projector.to_ardupilot_horiz_accuracy_m(output)
+        wgs = self._extract_wgs84(output)
+        emitted_at = time.monotonic_ns()
+        self._sequence_number += 1
+        seq = self._sequence_number
+        try:
+            self._connection.mav.gps_input_send(
+                int(self._clock_us()),
+                0,  # gps_id (primary)
+                0,  # ignore_flags
+                0,  # time_week_ms
+                0,  # time_week
+                _GPS_FIX_TYPE_3D,
+                int(wgs.lat_deg * 1e7),
+                int(wgs.lon_deg * 1e7),
+                float(wgs.alt_m),
+                0.0,  # hdop
+                0.0,  # vdop
+                0.0,  # vn
+                0.0,  # ve
+                0.0,  # vd
+                0.0,  # speed_accuracy
+                float(horiz_accuracy_m),
+                0.0,  # vert_accuracy
+                10,  # satellites_visible (synthetic; cosmetic for AP EKF)
+                0,  # yaw
+            )
+            self._connection.mav.named_value_float_send(
+                int(self._clock_ms_boot()),
+                _NAMED_VALUE_FLOAT_NAME.encode("utf-8"),
+                source_label_to_float(output.source_label),
+            )
+        except Exception as exc:
+            self._log_emit_failed(repr(exc), output.frame_id)
+            raise FcEmitError(f"AP outbound wire emit failed: {exc!r}") from exc
+        # AC-NEW-4 / AZ-395 AC-6: poll signing-failure counter post-emit;
+        # only log on the transition past the configured threshold.
+        self._poll_signing_failure_counter()
+        # Source-label transition STATUSTEXT (AC-4).
+        self._provenance.note_label_and_maybe_emit(output.source_label, severity=Severity.INFO)
+        if not self._first_emit_logged:
+            self._first_emit_logged = True
+            self._log.info(
+                "c8.ap.first_emit",
+                extra={
+                    "kind": "c8.ap.first_emit",
+                    "kv": {"frame_id": output.frame_id, "seq": seq},
+                },
+            )
+        self._log.debug(
+            "c8.ap.emit",
+            extra={
+                "kind": "c8.ap.emit",
+                "kv": {
+                    "frame_seq": seq,
+                    "horiz_accuracy_m": horiz_accuracy_m,
+                    "source_label": output.source_label,
+                },
+            },
+        )
+        return EmittedExternalPosition(
+            fc_kind=FcKind.ARDUPILOT_PLANE,
+            horiz_accuracy_m=horiz_accuracy_m,
+            source_label=output.source_label,
+            emitted_at=emitted_at,
+            sequence_number=seq,
+        )
+
+    def emit_status_text(self, msg: str, severity: Severity) -> None:
+        if not self._opened or self._connection is None:
+            raise FcEmitError("adapter not opened")
+        self._enforce_single_writer()
+        self._send_statustext_internal(msg, severity)
+
+    def request_source_set_switch(self) -> None:
+        raise NotImplementedError("Owned by source-set task; install AZ-396 to enable")
+
+    def current_flight_state(self) -> FlightStateSignal:
+        if self._inbound is None:
+            raise FcEmitError("current_flight_state requires open(); no inbound decoder yet")
+        latest = self._inbound.state_ring.peek_latest()
+        if latest is None:
+            return FlightStateSignal(
+                state=FlightState.INIT,
+                last_valid_gps_hint_wgs84=None,
+                last_valid_gps_age_ms=None,
+                captured_at=time.monotonic_ns(),
+            )
+        payload = latest.payload
+        assert isinstance(payload, FlightStateSignal)
+        return payload
+
+    # ------------------------------------------------------------------
+    # Internals
+
+    def _enforce_single_writer(self) -> None:
+        cur = threading.get_ident()
+        if self._open_emit_thread_ident is None:
+            self._open_emit_thread_ident = cur
+            return
+        if self._open_emit_thread_ident != cur:
+            raise RuntimeError(
+                "PymavlinkArdupilotAdapter outbound is single-writer; "
+                f"first thread={self._open_emit_thread_ident}, this thread={cur}"
+            )
+
+    def _connect(self, port: PortConfig) -> Any:
+        if self._connect_factory is not None:
+            return self._connect_factory(port.device, port.baud)
+        # Lazy import so the test path with an injected factory does NOT
+        # require pymavlink at module-import time.
+        from pymavlink import mavutil
+
+        return mavutil.mavlink_connection(
+            port.device,
+            baud=port.baud,
+            dialect="common",
+            mavlink_version="2.0",
+        )
+
+    def _setup_signing(self, key: bytearray) -> None:
+        if not hasattr(self._connection, "setup_signing"):
+            return
+        # pymavlink's setup_signing accepts bytes; pass the buffer (it
+        # converts to bytes internally so the buffer here stays the
+        # authoritative copy we zeroise on close).
+        self._connection.setup_signing(bytes(key))
+
+    def _materialise_signing_key(self, supplied: bytes | None) -> bytearray:
+        if supplied is None:
+            return bytearray(secrets.token_bytes(_SIGNING_KEY_LEN))
+        if len(supplied) != _SIGNING_KEY_LEN:
+            raise SigningHandshakeError(
+                f"signing_key must be {_SIGNING_KEY_LEN} bytes; got {len(supplied)}"
+            )
+        return bytearray(supplied)
+
+    def _materialise_dev_static_key(self) -> bytearray:
+        """Hex-decode FcConfig.dev_static_signing_key into a 32-byte buffer."""
+        hex_str = self._config.fc.dev_static_signing_key.strip()
+        try:
+            raw = bytes.fromhex(hex_str)
+        except ValueError as exc:
+            raise SigningHandshakeError(
+                "FcConfig.dev_static_signing_key must be valid hex"
+            ) from exc
+        if len(raw) != _SIGNING_KEY_LEN:
+            raise SigningHandshakeError(
+                f"FcConfig.dev_static_signing_key decodes to {len(raw)} bytes; "
+                f"expected {_SIGNING_KEY_LEN}"
+            )
+        return bytearray(raw)
+
+    @staticmethod
+    def _zeroise_signing_key(buf: bytearray) -> None:
+        for i in range(len(buf)):
+            buf[i] = 0
+
+    def _handle_signing_handshake_failure(self, exc: BaseException) -> None:
+        self._log.error(
+            f"c8.ap.signing_handshake_failed: {exc!r}",
+            extra={
+                "kind": "c8.ap.signing_handshake_failed",
+                "kv": {"error_class": type(exc).__name__},
+            },
+        )
+        self._fdr_signing_event(
+            kind="c8.ap.signing_handshake_failed",
+            kv={
+                "flight_id": self._flight_id,
+                "error_class": type(exc).__name__,
+            },
+        )
+
+    def _poll_signing_failure_counter(self) -> None:
+        if self._signing_key is None:
+            return
+        count = getattr(self._connection.mav, "signing", None)
+        if count is None:
+            return
+        failure_count = int(getattr(count, "sig_count", 0) or 0)
+        if failure_count <= self._signing_failure_logged_at_count:
+            return
+        if failure_count < self._signing_failure_threshold:
+            return
+        # Transition past the threshold — log + FDR + STATUSTEXT, no raise.
+        self._signing_failure_logged_at_count = failure_count
+        self._log.error(
+            f"c8.ap.signing_failure: count={failure_count}",
+            extra={
+                "kind": "c8.ap.signing_failure",
+                "kv": {"failure_count": failure_count},
+            },
+        )
+        self._fdr_signing_event(
+            kind="c8.ap.signing_failure",
+            kv={
+                "flight_id": self._flight_id,
+                "failure_count": failure_count,
+            },
+        )
+        try:
+            self._send_statustext_internal(
+                f"signing failure count={failure_count}", Severity.WARNING
+            )
+        except Exception:
+            # STATUSTEXT failure on the same UART is non-fatal; the
+            # ERROR log + FDR already captured the event.
+            pass
+
+    def _send_statustext_internal(self, msg: str, severity: Severity) -> None:
+        if self._connection is None:
+            return
+        try:
+            text = msg.encode("utf-8")[:50]
+            self._connection.mav.statustext_send(_mav_severity(severity), text)
+        except Exception as exc:
+            self._log.debug(
+                f"c8.ap.statustext_failed: {exc!r}",
+                extra={
+                    "kind": "c8.ap.statustext_failed",
+                    "kv": {"error": repr(exc)},
+                },
+            )
+
+    def _log_emit_failed(self, reason: str, frame_id: int) -> None:
+        self._log.error(
+            f"c8.ap.emit_failed: {reason}",
+            extra={
+                "kind": "c8.ap.emit_failed",
+                "kv": {"reason": reason, "frame_id": frame_id},
+            },
+        )
+
+    def _extract_wgs84(self, output: EstimatorOutput) -> LatLonAlt:
+        """Pull the WGS84 fix the composition root pre-attached.
+
+        C5 emits its estimate in the local ENU frame; the composition
+        root injects a WgsConverter-backed enricher that attaches the
+        WGS84 conversion to ``output.extras["wgs84"]`` BEFORE handing
+        the output to C8. If the enricher is missing the wgs84 key,
+        that is a composition bug — fail loudly rather than guess.
+        """
+        wgs = output.extras.get("wgs84") if output.extras else None
+        if not isinstance(wgs, LatLonAlt):
+            raise FcEmitError(
+                "EstimatorOutput.extras['wgs84'] missing or not a LatLonAlt; "
+                "composition root must inject the ENU->WGS84 enricher"
+            )
+        return wgs
+
+    def _clock_us(self) -> int:
+        return int(self._clock() * 1_000_000)
+
+    def _clock_ms_boot(self) -> int:
+        return int(self._clock() * 1_000)
+
+    def _fdr_signing_event(self, *, kind: str, kv: dict[str, Any]) -> None:
+        record = FdrRecord(
+            schema_version=1,
+            ts=datetime.now(tz=timezone.utc).isoformat(),
+            producer_id="c8_fc_adapter",
+            kind="log",
+            payload={
+                "level": "INFO" if kind.endswith("_rotated") else "ERROR",
+                "component": "c8_fc_adapter",
+                "kind": kind,
+                "msg": kind,
+                "kv": kv,
+            },
+        )
+        try:
+            self._fdr_client.enqueue(record)
+        except Exception as exc:
+            self._log.debug(
+                f"c8.ap.fdr_enqueue_failed: {exc!r}",
+                extra={
+                    "kind": "c8.ap.fdr_enqueue_failed",
+                    "kv": {"error": repr(exc), "downstream_kind": kind},
+                },
+            )