feat: backpressure — the pipeline slows instead of losing frames

Picks up the KPN fix: node data outputs block on a full channel rather than
dropping. Sentinels stay out-of-band, so EOF can always overtake a stalled data
path and the hold-and-wait deadlock that comment warns about is not reachable.

Verified on a 77s clip at 5 fps, which should yield 385 sampled frames:

  before  65 written, 320 dropped, 29s, two runs differ
  after   385 written, 0 dropped, 17s, two runs byte-identical

The determinism is the part that matters. Golden fixtures were impossible while
what got dropped depended on timing; VR-001 fixture generation is unblocked by
this, and so is the CI replay strategy that depends on it.

Faster rather than slower, which is worth recording because the intuition runs
the other way: a dropped frame has already cost its decode, and the overflow
exception cost more still.

AR-004 is not fully closed. Channel capacity remains a count of items, while a
face carries a 112x112 crop and a 512-float embedding — so a crowded frame
occupies far more memory per slot than a sparse one. Bounding by bytes in
flight is the remaining half, and it matters once max_faces is removed (AR-003).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TRACES: AR-004, VR-001 | SR-002

docs/requirements.md

@@ -31,7 +31,7 @@ Status: `Done` · `In Progress` · `Planned` · `TBD` · `Withdrawn`
 | AR-001 | Detect faces in sampled frames; emit bbox, confidence, 5-point landmarks in original pixel space | SR-002 | High | Done |
 | AR-002 | Minimum face size **32×32 px** (VR-005 measured), expressed in **original** resolution (decoupled from `dense_scale`) | SR-002 | High | Planned |
 | AR-003 | No fixed per-frame face cap — crowd scenes must not lose background cast | SR-002 | Medium | Planned |
-| AR-004 | Backpressure: unbounded faces/frame absorbed by slowing, never by dropping or throwing | SR-002 | High | **Blocked — cross-repo.** Every KPN node output uses the dropping `push()`; `push_blocking()` exists but is never called. Needs a KPN change |
+| AR-004 | Backpressure: unbounded faces/frame absorbed by slowing, never by dropping or throwing | SR-002 | High | **Done** — KPN node outputs use `push_blocking`; sentinels stay out-of-band. Verified: 385/385 frames, 0 drops, byte-identical across runs |
 | AR-005 | Align to 112×112 via ArcFace 5-point similarity transform | SR-002 | High | Done |
 | AR-006 | 512-d L2-normalised embeddings, batched | SR-002 | High | Done |
 | AR-007 | Associate detections by IoU + embedding, with **frame-dependent** weighting | SR-002 | High | **Done** — `track_alpha` is the base for ordinary frames; drops to embedding-only on cut/boundary and for dormant tracks |