[AZ-401] [AZ-400] Replay — compose_root replay-mode branch + transport seam

Wires the airborne composition root for replay-as-configuration (ADR-011):

- compose_root(config) branches on config.mode in {"live", "replay"}.
  Live behaviour is unchanged; replay builds ReplayInputAdapter,
  attaches JsonlReplaySink, and injects NoopMavlinkTransport.
- New private module runtime_root/_replay_branch.py holds the
  replay-only strategy graph + build-flag gate + calibration loader.
- Config gains Config.mode (Literal["live","replay"]) plus
  Config.replay sub-block with nested ReplayAutoSyncConfig that mirrors
  the AZ-405 AutoSyncConfig DTO; YAML loader + ENV map updated.

Absorbs the AZ-400 transport-seam retrofit that AZ-401 strictly
required but AZ-400 had not delivered:

- New MavlinkTransport Protocol (write/bytes_written/close).
- NoopMavlinkTransport (replay; build-flag gated, idempotent close,
  thread-safe byte counter).
- SerialMavlinkTransport (live, no-op restructure of existing pymavlink
  byte path; encoder retrofit to actually USE it is the AZ-558
  follow-up).

AZ-401 AC-9 (NoopMavlinkTransport.bytes_written > 0 after C8 encoders
run) is BLOCKED on AZ-558 — the encoder routing retrofit is out of
the AZ-401 task envelope (FORBIDDEN files: pymavlink_ardupilot_adapter,
msp2_inav_adapter). AZ-558 spec, batch_61_review.md, and the test's
@pytest.mark.skip rationale all carry the deferral reason.

Tests: 22 compose_root replay-branch tests + 17 transport tests.
Full regression: 2063 passed, 86 environment-skips, 1 documented
skip (AC-9 / AZ-558), 1 pre-existing flaky perf test deselected.

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

tests/unit/c8_fc_adapter/test_smoke.py

@@ -1,4 +1,9 @@
-"""C8 FC Adapter smoke test — AC-9 (legacy) + AZ-390 public-API gate."""
+"""C8 FC Adapter smoke test — AC-9 (legacy) + AZ-390 public-API gate.
+
+AZ-401 expands the public Protocol surface with ``MavlinkTransport``,
+the outbound byte-stream seam shared by ``SerialMavlinkTransport``
+(live) and ``NoopMavlinkTransport`` (replay).
+"""
 
 
 def test_interface_importable() -> None:
@@ -7,10 +12,17 @@ def test_interface_importable() -> None:
         EmittedExternalPosition,
         FcAdapter,
         GcsAdapter,
+        MavlinkTransport,
         ReplaySink,
     )
 
-    for sym in (FcAdapter, GcsAdapter, ReplaySink, EmittedExternalPosition):
+    for sym in (
+        FcAdapter,
+        GcsAdapter,
+        ReplaySink,
+        EmittedExternalPosition,
+        MavlinkTransport,
+    ):
         assert sym is not None
 
 
@@ -24,5 +36,6 @@ def test_internal_modules_not_in_public_all() -> None:
         "EmittedExternalPosition",
         "FcAdapter",
         "GcsAdapter",
+        "MavlinkTransport",
         "ReplaySink",
     }