[AZ-558] Route C8 outbound encoder bytes through MavlinkTransport seam

All FC adapter outbound MAVLink bytes now go through the AZ-401
MavlinkTransport seam (NoopMavlinkTransport in replay,
SerialMavlinkTransport in live). New helpers in
_outbound_mavlink_payloads.py extract encode/pack/seq-bump so the four
AP _send sites and the iNav statustext _send site become
encode -> pack -> transport.write. TlogReplayFcAdapter emits real
AP-shape MAVLink bytes through the injected NoopMavlinkTransport,
satisfying replay protocol Invariant 5 and unblocking AZ-401 AC-9.

Closes AZ-558. Also unskips AZ-401 AC-9 and AZ-404 AC-4b. Live wire
output remains byte-identical (proven via two-instance MAVLink
byte-equivalence tests). AST scan asserts no .mav.<name>_send( calls
remain in the retrofit set (AP / iNav / tlog adapters).

Out of scope (logged in review): GCS adapter retrofit; airborne live
strategy registration that would activate the SerialMavlinkTransport
factory injection path.

Tests: 2110 passed, 92 environmental skips, 1 unrelated pre-existing
macOS cold-start flake deselected.

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

tests/unit/c8_fc_adapter/test_az393_ardupilot_outbound.py

@@ -35,26 +35,43 @@ from gps_denied_onboard.components.c8_fc_adapter.pymavlink_ardupilot_adapter imp
 )
 from gps_denied_onboard.config import load_config
 
+from ._mav_test_helpers import _FakeMsg
+
 # ----------------------------------------------------------------------
 # Helpers — pymavlink stand-in
 
 
+class _SigningStub:
+    sign_outgoing = False
+
+
 class _MavStub:
-    """Captures pymavlink ``mav.*_send`` calls for wire-level assertions."""
+    """Captures pymavlink ``mav.*_encode`` calls for wire-level assertions.
+
+    Post-AZ-558 the AP adapter routes through ``encode → pack → transport.write``;
+    we record args at the ``encode`` boundary (where the test cares),
+    return a :class:`_FakeMsg` whose ``pack`` produces placeholder bytes,
+    and the transport seam swallows them.
+    """
 
     def __init__(self) -> None:
         self.gps_input_calls: list[tuple[Any, ...]] = []
         self.named_value_float_calls: list[tuple[Any, ...]] = []
         self.statustext_calls: list[tuple[int, bytes]] = []
+        self.seq: int = 0
+        self.signing = _SigningStub()
 
-    def gps_input_send(self, *args: Any) -> None:
+    def gps_input_encode(self, *args: Any) -> _FakeMsg:
         self.gps_input_calls.append(args)
+        return _FakeMsg()
 
-    def named_value_float_send(self, time_boot_ms: int, name: bytes, value: float) -> None:
+    def named_value_float_encode(self, time_boot_ms: int, name: bytes, value: float) -> _FakeMsg:
         self.named_value_float_calls.append((time_boot_ms, name, value))
+        return _FakeMsg()
 
-    def statustext_send(self, severity: int, text: bytes) -> None:
+    def statustext_encode(self, severity: int, text: bytes) -> _FakeMsg:
         self.statustext_calls.append((severity, text))
+        return _FakeMsg()
 
 
 class _ConnStub:
@@ -62,10 +79,15 @@ class _ConnStub:
         self.mav = _MavStub()
         self.setup_signing_calls: list[bytes] = []
         self.closed = False
+        self.write_calls: list[bytes] = []
 
     def setup_signing(self, key: bytes) -> None:
         self.setup_signing_calls.append(bytes(key))
 
+    def write(self, payload: bytes) -> int:
+        self.write_calls.append(bytes(payload))
+        return len(payload)
+
     def close(self) -> None:
         self.closed = True