docs: bali fixture corpus, and AR-004 blocks reproducible fixtures

Records `bali/` — five ~77s clips of Road to Bali (1952) — as the fixture
source. Public domain, which is the point rather than a convenience: derived
fixtures can be committed, where anything cut from a copyrighted title could
not live in the repository at all.

Makes explicit what the tier table only implied: CI never calls a model. Not a
preference — the embedder measures ~930 ms/frame on the CPU provider, so a 77s
clip at 5 fps is six minutes of inference. Every model invocation happens
locally and CI consumes the result as data, which is what makes the T1/T2 split
load-bearing rather than stylistic.

Two properties of the corpus to design around: 480x360 puts many faces below
the AR-002 66px minimum, so generation must set and record --min-face-px; and
77s at 1 fps is too thin to exercise an extinction window measured in tens of
seconds, so fixtures want 5 fps.

The finding that matters: a trial dump produced 49 frames of an expected ~385,
stopping at 51s of 77s, with 285 frames dropped at camera_pos and 51 at
face_aligner on channel overflow. Channels drop rather than block, and what
drops depends on timing, so the same command twice can yield different dumps.
Golden fixtures cannot be built on that — AR-004 is a prerequisite for VR-001
fixtures, not just a throughput concern for crowd scenes.

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

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

docs/requirements.md

@@ -213,6 +213,46 @@ structurally unverifiable on the CI host. It needs a GPU host and a synthetic
 large gallery, so it is the requirement most likely to silently regress. Its
 benchmark (VR-008) should run on a schedule rather than on demand.
 
+### CI never calls a model
+
+**Not "should not" — cannot.** The N100 has no GPU, and even the ONNX Runtime CPU
+provider is impractical: a measured run of the embedder on this hardware sits at
+~930 ms per frame, so a 77 s clip at 5 fps would take roughly six minutes of
+inference alone. Every model invocation therefore happens **locally, ahead of
+time**, and CI consumes the result as data.
+
+This is what makes the T1/T2 split load-bearing rather than a preference: T1 and
+T2 are the only tiers that can exist in CI at all.
+
+### Fixture corpus — `bali/`
+
+Five clips of **Road to Bali (1952)**, ~77 s each, 480×360, 30 fps, 42 MB total.
+
+Public domain, and that is the reason to use it rather than a convenience:
+**derived fixtures — dumps, crops, golden outputs — can be committed without the
+rights question that rules out sharing gallery data (SR-005).** A fixture cut
+from a copyrighted title could not live in the repository at all.
+
+Two properties to design around rather than discover:
+
+- **480×360 means small faces.** At this resolution a face is often 40–80 px, so
+  the AR-002 minimum of 66 px (original resolution) rejects much of what is
+  there. Fixture generation must set `--min-face-px` explicitly and record it,
+  or the dumps will be sparse for reasons unrelated to what is being tested.
+- **77 s is short.** At 1 fps that is 77 frames — too thin to exercise an
+  extinction window measured in tens of seconds. Generate at 5 fps (≈385 frames,
+  ~1 MB) and record the rate in provenance, since the behaviour under test
+  changes with it.
+
+> **AR-004 blocks reproducible fixture generation.** A trial run of one clip
+> produced 49 frames of an expected ~385, ending at 51 s of 77 s, with the
+> diagnostics reporting 285 frames dropped at `camera_pos` and 51 at
+> `face_aligner`. Channels overflow and **drop** rather than blocking, and what
+> gets dropped depends on timing — so the same command run twice can produce
+> different dumps. Golden fixtures cannot be built on that. AR-004 is therefore
+> a prerequisite for VR-001 fixtures, not merely a throughput concern for crowd
+> scenes.
+
 ### Fixtures — precomputed inference, pulled by CI
 
 The N100 cannot run inference at any useful rate, so **inference output is