[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/runtime_root/_replay_branch.py

@@ -126,10 +126,22 @@ def build_replay_components(
 
     sink_fdr_client = fdr_factory("c8_fc_adapter.replay_sink", config)
 
+    # AZ-558: build the outbound MAVLink transport BEFORE the FC adapter
+    # so it can be threaded through `ReplayInputAdapter` and into
+    # `TlogReplayFcAdapter`. The same instance is exposed as the
+    # ``mavlink_transport`` slot in ``components`` (replay protocol
+    # Invariant 5: encoders write through the seam in both modes;
+    # replay drops the bytes via NoopMavlinkTransport).
+    if transport_factory is not None:
+        transport = transport_factory(config)
+    else:
+        transport = NoopMavlinkTransport()
+
     bundle = _build_replay_input_bundle(
         config,
         fdr_client=fdr_client,
         adapter_factory=replay_input_adapter_factory,
+        mavlink_transport=transport,
     )
 
     if sink_factory is not None:
@@ -140,11 +152,6 @@ def build_replay_components(
             fdr_client=sink_fdr_client,
         )
 
-    if transport_factory is not None:
-        transport = transport_factory(config)
-    else:
-        transport = NoopMavlinkTransport()
-
     components: dict[str, Any] = {
         "frame_source": bundle.frame_source,
         "fc_adapter": bundle.fc_adapter,
@@ -188,6 +195,7 @@ def _build_replay_input_bundle(
     *,
     fdr_client: "FdrClient",
     adapter_factory: Any | None,
+    mavlink_transport: Any | None = None,
 ) -> ReplayInputBundle:
     """Build the :class:`ReplayInputAdapter` and call ``open()``."""
     pace = _resolve_pace(config.replay.pace)
@@ -205,6 +213,7 @@ def _build_replay_input_bundle(
             fdr_client=fdr_client,
             pace=pace,
             auto_sync_config=auto_sync,
+            mavlink_transport=mavlink_transport,
         )
     else:
         adapter = ReplayInputAdapter(
@@ -217,6 +226,7 @@ def _build_replay_input_bundle(
             pace=pace,
             manual_time_offset_ms=config.replay.time_offset_ms,
             auto_sync_config=auto_sync,
+            mavlink_transport=mavlink_transport,
         )
     return adapter.open()