test: replay the real tracker and registry from committed fixtures

Tier T2 — composition rather than units. The registry tests construct awkward
states directly; these feed the pieces real 480x360 footage with the cuts, gaps
and crowded frames that synthetic input does not produce.

Six cases:
- fixture integrity: exact frame and face counts, contiguous face_offset, and
  the embedder identity each dump carries (GR-004). The counts are asserted
  exactly rather than approximately, which was impossible before AR-004 — what
  a lossy run dropped depended on timing.
- determinism: replaying a fixture twice gives identical track ids and windows.
  This is the property the whole fixture strategy rests on; without it every
  golden output derived from a fixture is unreliable and the CI replay tier is
  worthless.
- every face is assigned a track, and flush leaves nothing open — a track still
  live at EOF is a window that never reaches the output.
- windows are well-formed and inside the clip. A window ends at the last
  sighting, so it can never extend past the footage that produced it.
- a longer extinction window yields fewer, longer tracks. On the sparse fixture
  (140 faces over 385 frames) that is the difference the constant actually
  makes: absorbing a gap versus splitting a window.
- the cut-heavy fixture still contains cuts. This guards the corpus, not the
  code: a regeneration that produced cut-free fixtures would leave the
  association tests passing while silently testing nothing.

Driving the functors directly rather than through a KPN network is deliberate —
no threads, no channels, no scheduling, so the same input gives the same output.

Suite: 86 cases, 6106 assertions.

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

TRACES: AR-004, AR-012, AR-013, VR-001, VR-002 | SR-002

docs/traceability.md

@@ -3,7 +3,7 @@
 <!-- GENERATED FILE - do not edit by hand. -->
 <!-- Regenerate: scripts/traceability/traceability-gate.sh -->
 
-**Generated:** 2026-07-31T08:10:51+00:00
+**Generated:** 2026-07-31T08:35:29+00:00
 
 Denominators are read from [`requirements.md`](requirements.md) at run time, never hardcoded. Coverage counts a requirement only when it is tagged in source **and** has a verification tier this repo's CI host can execute (`T1, T2, T3, static`).
 
@@ -78,9 +78,9 @@ _None._
 | ID | Status | Tier | Traces to | Trace state | Tagged in | Requirement |
 |---|---|---|---|---|---|---|
 | AR-001 | Done | T3 | SR-002 | covered | `src/nodes/face_detector_node.hpp` | Detect faces in sampled frames; emit bbox, confidence, 5-point landma… |
-| AR-002 | Planned | T2 | SR-002 | untagged | - | Minimum face size 66×66 px, expressed in **original** resolution (dec… |
+| AR-002 | Planned | unset | SR-002 | untagged | - | Minimum face size **32×32 px** (VR-005 measured), expressed in **orig… |
 | AR-003 | Planned | T1, T2, T4 | SR-002 | untagged | - | No fixed per-frame face cap — crowd scenes must not lose background c… |
-| AR-004 | Planned | T1, T4 | SR-002 | untagged | - | Backpressure: unbounded faces/frame absorbed by slowing, never by dro… |
+| AR-004 | **Done** — KPN node… | T1, T4 | SR-002 | untagged | - | Backpressure: unbounded faces/frame absorbed by slowing, never by dro… |
 | AR-005 | Done | T1, T3 | SR-002 | covered | `src/face_utils.hpp` | Align to 112×112 via ArcFace 5-point similarity transform |
 | AR-006 | Done | T3 | SR-002 | untagged | - | 512-d L2-normalised embeddings, batched |
 | AR-007 | **Done** — `track_a… | T2 | SR-002 | covered | `src/config.hpp`, `src/main.cpp`, `src/nodes/face_tracker_node.hpp` | Associate detections by IoU + embedding, with **frame-dependent** wei… |
@@ -133,7 +133,7 @@ _None._
 | VR-002 | Done | out-of-ci | PR-002 | tagged, unexecuted | `scripts/optimizer/replay.py` | Replay drives the **real** KPN nodes, not a reimplementation |
 | VR-003 | Done | out-of-ci | PR-002 | tagged, unexecuted | `scripts/optimizer/second_score.py` | Scoring: micro-F1 against X-Ray, precision/recall logged at every eva… |
 | VR-004 | Done | out-of-ci | PR-002 | untagged | - | Reproducible validation corpus with ground truth |
-| VR-005 | Planned | out-of-ci | PR-002 | untagged | - | Minimum face size study — TPI/FPI vs probe size, gallery held at nati… |
+| VR-005 | **Done** — knee at … | out-of-ci | PR-002 | untagged | - | Minimum face size study — TPI/FPI vs probe size, gallery held at nati… |
 | VR-006 | Planned | out-of-ci | PR-002 | untagged | - | Re-tune `scene_threshold` once native-rate decode lands |
 | VR-007 | Planned | out-of-ci | PR-002 | untagged | - | Expansion band, clustering threshold, and deferred-pass ablation |
 | VR-008 | Planned | out-of-ci | PR-002 | untagged | - | Gallery scaling benchmark — throughput vs gallery size |