[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_az394_inav_outbound.py

@@ -35,6 +35,8 @@ from gps_denied_onboard.components.c8_fc_adapter.msp2_inav_adapter import (
 )
 from gps_denied_onboard.config import load_config
 
+from ._mav_test_helpers import _FakeMsg
+
 # ----------------------------------------------------------------------
 # Helpers — MSP / secondary-MAVLink stand-ins
 
@@ -51,22 +53,35 @@ class _MspStub:
         self.closed = True
 
 
+class _SigningStub:
+    sign_outgoing = False
+
+
 class _SecondaryMavStub:
     def __init__(self) -> None:
         self.statustext_calls: list[tuple[int, bytes]] = []
         self.closed = False
+        self.write_calls: list[bytes] = []
         # Mirror pymavlink connection.mav shape.
         self.mav = self
         # Track signing-key state per RESTRICT-COMM-2 (Invariant 9):
         # the unit-test adapter MUST never call setup_signing on us.
         self.setup_signing_calls: list[Any] = []
+        # AZ-558: encoder flow needs ``mav.seq`` and ``mav.signing``.
+        self.seq: int = 0
+        self.signing = _SigningStub()
 
-    def statustext_send(self, severity: int, text: bytes) -> None:
+    def statustext_encode(self, severity: int, text: bytes) -> _FakeMsg:
         self.statustext_calls.append((int(severity), bytes(text)))
+        return _FakeMsg()
 
     def setup_signing(self, key: Any) -> None:
         self.setup_signing_calls.append(key)
 
+    def write(self, payload: bytes) -> int:
+        self.write_calls.append(bytes(payload))
+        return len(payload)
+
     def close(self) -> None:
         self.closed = True