[AZ-291] [AZ-292] [AZ-293] C13 FDR writer chain (batch 6)

AZ-291 — FileFdrWriter: single writer thread draining every registered
FdrClient SPSC ring buffer to per-flight segment files; per-segment
size rotation; cross-process fcntl.flock filelock on flight_root;
ENOSPC degraded mode with rate-capped ERROR logs and one GCS alert.

AZ-292 — FlightHeader/FlightFooter dataclasses + open_flight /
close_flight lifecycle methods; four per-flight monotonic counters
(records_written, records_dropped_overrun, bytes_written,
rollover_count) reported by the footer; flight_id mismatch and
close-without-open are typed errors.

AZ-293 — CapacityCapPolicy (post-rotation hook): walks the flight
directory, drops the oldest CLOSED segment when total > cap (default
64 GiB), emits a kind="segment_rollover" record per drop. Never drops
the currently-open segment or segment 0 alone; cap_misconfigured path
logs ERROR + GCS alert. No config flag disables emission (C13-ST-01).

Schema: bumped fdr_record_schema flight_header / flight_footer payload
key sets to match the AZ-292 task spec (effective 1.0.0 -> 1.1.0; no
prior producer); KNOWN_PAYLOAD_KEYS updated. Added FdrWriterConfig
nested in FdrConfig (segment_size_bytes, batch_size, flight_cap_bytes,
debug_log_per_record).

Tests: 29 new unit tests (8 AC + 1 invariant per task); full suite
323 passed, 2 pre-existing skips, 0 regressions.

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

src/gps_denied_onboard/fdr_client/records.py

@@ -46,8 +46,29 @@ KNOWN_PAYLOAD_KEYS: Final[dict[str, frozenset[str]]] = {
     "segment_rollover": frozenset({"old_segment", "new_segment", "total_bytes_after"}),
     "failed_tile_thumbnail": frozenset({"frame_id", "tile_id", "jpeg_bytes_b64"}),
     "mid_flight_tile_snapshot": frozenset({"snapshot_path", "captured_at"}),
-    "flight_header": frozenset({"flight_id", "started_at", "schema_version", "build_info"}),
-    "flight_footer": frozenset({"flight_id", "ended_at", "records_written", "records_dropped"}),
+    "flight_header": frozenset(
+        {
+            "flight_id",
+            "flight_started_at_iso",
+            "flight_started_at_monotonic_ns",
+            "config_snapshot",
+            "signing_key_rotation_event",
+            "manifest_content_hashes",
+            "build_info",
+        }
+    ),
+    "flight_footer": frozenset(
+        {
+            "flight_id",
+            "flight_ended_at_iso",
+            "flight_ended_at_monotonic_ns",
+            "records_written",
+            "records_dropped_overrun",
+            "bytes_written",
+            "rollover_count",
+            "clean_shutdown",
+        }
+    ),
 }
 
 KNOWN_KINDS: Final[frozenset[str]] = frozenset(KNOWN_PAYLOAD_KEYS.keys())