docs: AR-004 is a KPN change, with the measurement behind it

Backpressure cannot be implemented in this repository. Every node output in KPN
uses the dropping push() (pool_node.hpp:404 and :710, plus branch, fanout and
interrupt_node). A lossless push_blocking() already exists on both Channel and
OutputPort — "wait for the consumer to drain instead of dropping; the producer
just runs slower" — and nothing calls it. The fix is a per-channel policy or a
network default in KPN, and this pipeline should select lossless: a dropped
frame here does not degrade a result, it silently changes one.

Measured rather than inferred. One 77s clip at 5 fps should yield ~385 sampled
frames. On CPU it produced 49, ending at 51s, with 285 dropped at camera_pos
and 51 at face_aligner. Rebuilt with CUDA the same clip ran in 29s and reached
EOF correctly, and still dropped 320 at camera_pos, yielding 65. Faster
hardware moves where the queue backs up; it does not change what happens when
it does — which is why this is a correctness requirement rather than a
throughput one.

Raising channel capacity is therefore a stopgap: it lowers the probability of
overflow without changing the behaviour on overflow, and the failure it hides
is silent corruption of the output.

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

TRACES: AR-003, 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@@ -31,7 +31,7 @@ Status: `Done` · `In Progress` · `Planned` · `TBD` · `Withdrawn`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 | 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-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 |