docs: minimum face size is 32px, measured (VR-005)

Replaces the 66px working estimate with the sweep result. 258 probes degraded
to each size and matched against a native-resolution gallery: 16px 46.5% TPI,
24px 93.4%, 32px 98.1%, 40px 99.2%, flat to 112px. The knee is 24-32 and 32
sits within about a point of the ceiling.

The estimate was roughly twice too strict. At 66px a large share of usable
faces would have been discarded, and on 480x360 sources most of them — which
is exactly the resolution of the fixture corpus.

The more useful finding: false identification was 0.0 at every size, including
12px. Small faces fail by becoming unidentified, never by being attributed to
the wrong actor. That asymmetry is what makes a low threshold safe — the cost
of admitting a marginal face is a miss, not a false claim.

Caveat recorded rather than assumed: FPI grows with gallery size, so 258 actors
understates it against a full library. Treat 0.0 as an observation at this
scale, not a property.

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

TRACES: AR-002, VR-005 | SR-002, PR-002

docs/requirements.md

@@ -29,7 +29,7 @@ Status: `Done` · `In Progress` · `Planned` · `TBD` · `Withdrawn`
 | ID | Requirement | Traces to | Priority | Status |
 |---|---|---|---|---|
 | 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 66×66 px, expressed in **original** resolution (decoupled from `dense_scale`) | SR-002 | High | Planned |
+| 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-005 | Align to 112×112 via ArcFace 5-point similarity transform | SR-002 | High | Done |
@@ -104,7 +104,7 @@ Status: `Done` · `In Progress` · `Planned` · `TBD` · `Withdrawn`
 | VR-002 | Replay drives the **real** KPN nodes, not a reimplementation | PR-002 | High | Done |
 | 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 | Planned |
+| 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 |
 | VR-006 | Re-tune `scene_threshold` once native-rate decode lands | PR-002 | Low | Planned |
 | VR-007 | Expansion band, clustering threshold, and deferred-pass ablation | PR-002 | Medium | Planned |
 | VR-008 | Gallery scaling benchmark — throughput vs gallery size | PR-002 | Medium | Planned |
@@ -195,7 +195,7 @@ as such rather than counted as covered.
 | Requirement | Tier | Note |
 |---|---|---|
 | AR-001, AR-005, AR-006 | T3 | Smoke only — correctness of detection/embedding is a model property, not ours |
-| AR-002 | T2 | Size filtering is arithmetic on dumped bboxes |
+| AR-002 | Minimum face size **32×32 px** (VR-005 measured), expressed in **original** resolution (decoupled from `dense_scale`) | Size filtering is arithmetic on dumped bboxes |
 | AR-003, AR-004 | T1 + T4 | Backpressure logic is unit-testable; saturation behaviour needs real load |
 | AR-007 … AR-017 | **T2** | The core of the redesign — fully replayable |
 | AR-018 … AR-022 | **T2** | Expansion, deferred pass, clustering: all post-embedding |
@@ -302,7 +302,7 @@ because it will be trusted.
 | ID | Tier | Test asserts | Edge cases to cover |
 |---|---|---|---|
 | AR-001 | T3 | Detector returns plausible boxes on a known frame | — smoke only |
-| AR-002 | T2 | Faces below 66 px (original res) are dropped | Exactly at threshold; with `dense_scale` 0.5 — the interaction that motivated the requirement |
+| AR-002 | Minimum face size **32×32 px** (VR-005 measured), expressed in **original** resolution (decoupled from `dense_scale`) | Faces below 66 px (original res) are dropped | Exactly at threshold; with `dense_scale` 0.5 — the interaction that motivated the requirement |
 | AR-003 | T2 | No cap applied; a 40-face frame yields 40 | Crowd frame |
 | AR-004 | T1 | Saturated input blocks rather than drops or throws | Bounded queue at capacity; **byte-based** limit with large crops; SIGTERM mid-block |
 | AR-005 | T1 | Known landmarks → expected 112×112 warp | Landmarks near frame edge; degenerate/collinear points |