[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>

src/gps_denied_onboard/replay_input/tlog_video_adapter.py

@@ -131,6 +131,7 @@ class ReplayInputAdapter:
         "_tlog_source_factory",
         "_video_frames_factory",
         "_video_timestamps_factory",
+        "_mavlink_transport",
         "_log",
         "_opened",
         "_closed",
@@ -152,6 +153,7 @@ class ReplayInputAdapter:
         tlog_source_factory: Any | None = None,
         video_frames_factory: Any | None = None,
         video_timestamps_factory: Any | None = None,
+        mavlink_transport: Any | None = None,
     ) -> None:
         if not isinstance(video_path, Path):
             raise ReplayInputAdapterError(
@@ -182,6 +184,7 @@ class ReplayInputAdapter:
         self._tlog_source_factory = tlog_source_factory
         self._video_frames_factory = video_frames_factory
         self._video_timestamps_factory = video_timestamps_factory
+        self._mavlink_transport = mavlink_transport
         self._log = logging.getLogger("replay_input.tlog_video_adapter")
         self._opened = False
         self._closed = False
@@ -268,6 +271,7 @@ class ReplayInputAdapter:
                 time_offset_ms=resolved_offset_ms,
                 pace=self._pace,
                 source_factory=self._tlog_source_factory,
+                mavlink_transport=self._mavlink_transport,
             )
             fc_adapter.open()
         except (FcOpenError, FcAdapterConfigError, FcAdapterError) as exc: