[AZ-658] frame_ingest H.264/265 decoder (NVDEC + sw fallback)

Wires a real ffmpeg-next 8.1 decoder into the frame_ingest lifecycle
loop. NVDEC is probed at runtime via h264_cuvid / hevc_cuvid; CUDA-less
hosts transparently fall back to software h264 / hevc. Each decoded
frame is stamped with capture_ts (taken at packet receipt) and
decode_ts (taken after decode returns) so movement_detector sees
accurate frame-arrival times. Single-frame decode errors are counted
toward decode_errors_total and dropped; the stream is never aborted.

Adds new public API on FrameIngestHandle: decoder_backend(),
decode_errors_total(), frames_decoded_total(), decode_ms_first_frame(),
decode_ms_p50(), decode_ms_p99(). Integration tests under
crates/frame_ingest/tests/decoder_pipeline.rs cover AC-1, AC-3, AC-4
end-to-end through the real FfmpegDecoder using libx264-encoded
synthetic streams; AC-2 positive (NVDEC selection) is opt-in via
--ignored on a CUDA host. AZ-657 lifecycle tests retained via a
StubDecoder.

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

_docs/_autodev_state.md

@@ -4,27 +4,27 @@
 flow: greenfield
 step: 7
 name: Implement
-status: between-batches
+status: in_progress
 sub_step:
-  phase: 0
-  name: batch-16-select
-  detail: ""
+  phase: 11
+  name: commit
+  detail: "batch 16 — AZ-658 awaiting commit + push approval"
 retry_count: 0
 cycle: 1
 tracker: jira
 
 ## Last Completed Batch
-batch: 15
-commit: ccf929a
-ticket: AZ-676 / AZ-677 / AZ-678 / AZ-679
-jira_status: In Testing (all 4 confirmed via read-back)
-pushed_to: origin/dev
-report: _docs/03_implementation/batch_15_cycle1_report.md
+batch: 16
+commit: pending
+ticket: AZ-658
+jira_status: In Progress (transition to In Testing pending commit)
+pushed_to: pending
+report: _docs/03_implementation/batch_16_cycle1_report.md
 cumulative_review: _docs/03_implementation/cumulative_review_batches_13-15_cycle1_report.md
 
 ## Process Leftovers
-- `_docs/_process_leftovers/2026-05-20_autopilot_clippy.md` — C5 replay
-- `_docs/_process_leftovers/2026-05-20_mission_executor_ac3_flake.md` — C6 fix recipe
+- `_docs/_process_leftovers/2026-05-20_autopilot_clippy.md` — out-of-scope for batch 16
+- `_docs/_process_leftovers/2026-05-20_mission_executor_ac3_flake.md` — out-of-scope for batch 16
 
 ## Cumulative Review Cadence
-Last cumulative: batches 13–15 (just produced). Next due: end of batch 18.
+Last cumulative: batches 13–15. Next due: end of batch 18.