[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>

_docs/_autodev_state.md

@@ -8,7 +8,7 @@ status: in_progress
 sub_step:
   phase: 14
   name: loop-next-batch
-  detail: "batch 5 of N committed"
+  detail: "batch 6 of N committed"
 retry_count: 0
 cycle: 1
 tracker: jira