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/SPEC.md

@@ -110,8 +110,19 @@ Two consequences worth stating:
   depends on timing. The same command run twice can produce different dumps, and
   a golden fixture cannot be built on that.
 
-**Gap:** entire requirement, and it is **cross-repo** — the drop-versus-block
-decision lives in KPN. Removing `max_faces` (AR-003) remains gated on it.
+**Current:** fixed in KPN — node data outputs use `push_blocking`, sentinels
+remain out-of-band so EOF can always overtake a stalled data path. Verified on
+the same clip: 385 of 385 sampled frames written, zero drops, and two
+consecutive runs byte-identical where previously they were not.
+
+It also ran *faster* (29 s → 17 s). A dropped frame has already cost its decode,
+and the overflow exception cost more — so the lossy path was paying for work it
+then discarded.
+
+**Gap:** the remaining half — bounding by **bytes in flight** rather than item
+count. Channel capacity is still a count of items, and a face carries a 112×112
+crop plus a 512-float embedding, so a crowded frame occupies far more memory per
+slot than a sparse one. That matters once `max_faces` is removed (AR-003).
 
 ## AR-005 — Face alignment and crop