[AZ-391] C8 inbound: MAVLink + MSP2 decoders + rings + bus + warm-start

Adds the C8 inbound producer side:
- TelemetryRing[T]: bounded drop-oldest ring; first-overflow INFO log
  + monotonic dropped_count.
- SubscriptionBus + SubscriptionHandle: synchronous fan-out, lock-
  released-before-callback to avoid deadlock; subscriber crash caught
  + DEBUG-logged so one bad subscriber cannot kill the decode loop.
- PymavlinkInboundDecoder: pymavlink-based AP decoder for RAW_IMU,
  SCALED_IMU2, ATTITUDE, GPS_RAW_INT, GPS2_RAW, HEARTBEAT, STATUSTEXT.
  Out-of-order drop (Invariant 7) per-kind WARN. STATUSTEXT spoofing
  sentinel promotes subsequent GPS to GpsStatus.SPOOFED within 5 s.
  AC-5.1 warm-start hint cached on first 3D+ fix; embedded into
  every FlightStateSignal.
- Msp2InavInboundDecoder: YAMSPy-based iNav polling decoder for IMU /
  attitude / GPS / flight-state. signed=False always (RESTRICT-COMM-2);
  GpsStatus.SPOOFED is unreachable on iNav.

Adds yamspy>=0.3.3 + pyserial>=3.5 to pyproject.toml.

Tests: 443 pass / 2 skip / 0 fail (+33 in batch 9).

Contract: no drift on fc_adapter_protocol.md v1.0.0; this batch
implements the inbound producer side without changing signatures.

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

pyproject.toml

@@ -27,6 +27,10 @@ dependencies = [
     "sqlalchemy>=2.0",
     "alembic>=1.13",
     "pymavlink>=2.4",
+    # iNav MSP2 wire decoder for C8 inbound (AZ-391). MSPy is request-response
+    # on a serial transport; AP uses pymavlink for the same role.
+    "yamspy>=0.3.3,<0.4",
+    "pyserial>=3.5",
     "requests>=2.31",
     "structlog>=24.1",
     "click>=8.1",