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/requirements.md

@@ -101,7 +101,7 @@ Status: `Done` · `In Progress` · `Planned` · `TBD` · `Withdrawn`
 | ID | Requirement | Traces to | Priority | Status |
 |---|---|---|---|---|
 | VR-001 | HDF5 post-inference dump at the embedded-frame boundary | PR-002 | High | Done |
-| VR-002 | Replay drives the **real** KPN nodes, not a reimplementation | PR-002 | High | Done |
+| VR-002 | Replay drives the **real** KPN nodes, not a reimplementation | PR-002 | High | **Done** — replay driven from committed fixtures in `tests/test_replay_fixtures.cpp`; determinism asserted |
 | VR-003 | Scoring: micro-F1 against X-Ray, precision/recall logged at every evaluation | PR-002 | High | Done |
 | VR-004 | Reproducible validation corpus with ground truth | PR-002 | High | Done |
 | VR-005 | Minimum face size study — TPI/FPI vs probe size, gallery held at native res | PR-002 | Medium | **Done** — knee at 24–32 px; 32 px gives 98.1% TPI, 0.0 FPI at every size |