docs: VR-013 and VR-014 results; AR-002 raised to 40px

Records study results and the requirement change that follows from them.

VR-013 measures minimum face size end to end — gallery from one recording,
probes from another — rather than by degrading an already-aligned crop. Holding
90% of the plateau needs ~50 px that way against VR-005's ~22 px, the gap being
detection and landmark error rather than the embedder. AR-002 therefore takes
40 px, not 32: VR-005 isolates the embedder and is an upper bound, and 32 admits
faces in the falling region. FPI stayed 0.0% at every scale, and the ceiling is
cross-view rather than resolution.

VR-014 exercises audio-signature offset recovery on real film audio instead of
the synthetic golden tone. Forty random in-cap offsets, every one recovered to
the nearest frame, worst error 46 ms against a 500 ms budget — and 46 ms is the
quantisation floor rather than a result, since offsets land on whole 92.88 ms
frames. The runtime/2 anchor is confirmed through head-trimmed files.

The soft spot VR-014 found is tier labelling, not accuracy: the score drops with
sub-frame misalignment, so 27 of 40 correct alignments were demoted to `loose`.
One frame of slack in the score restores all forty to `audio` with false matches
unmoved.

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

TRACES: AR-002, VR-005, VR-013, VR-014 | SR-002, SR-003

docs/plan.md

@@ -243,7 +243,9 @@ Context crops opt-in behind `--dump-unidentified-crops`.
 
 **Depends on:** nothing (AR-002, AR-011); AR-004 blocks AR-003.
 
-- **AR-002** — `min_face_px` → 66, expressed in original resolution.
+- **AR-002** — `min_face_px` stays **40** (VR-013 measured it end to end) but must
+  be expressed in original resolution rather than decoded-frame space. The value
+  is already right in `config.hpp`; the change is the coordinate space.
 - **AR-011** — feed TransNetV2 at native rate; derive the dedup window from
   source fps rather than the hardcoded `0.04 s`.
 - **AR-004** — backpressure. `kMaxFaces` (`identity_matcher_node.hpp:133`)
@@ -324,8 +326,67 @@ Windows carry belief and route; `extraction.*` gains `extinction_sec` and
 
 ## VR-005 — Minimum face size study
 
-**Depends on:** nothing. Standalone Python, no C++ contact. Produces the measured
-value replacing AR-002's 66 px estimate.
+**Depends on:** nothing. Standalone Python, no C++ contact. **Done** — knee at
+24–32 px. It measures the embedder with alignment held perfect, so it bounds the
+answer from below rather than setting it; AR-002's floor comes from **VR-013**,
+which sweeps input resolution end to end and lands at 40 px.
+
+## VR-013 — Cross-source identification probe
+
+**Depends on:** `sae_embed` exposing `detect()`, `align_face()`, `embed_crop()`
+and the gallery calibration — it drives the shipped C++ rather than reimplementing
+it, which is what VR-005 could not do.
+
+Gallery from one recording, probes from another, sweeping the probe's **input
+resolution before the detector**, so detection and landmark regression degrade
+with the frame. `experiments/xsource/`.
+
+**Findings.** Holding 90% of the plateau needs ~50 px end to end against VR-005's
+~22 px; `min_face_px` 40 is right and 32 would admit faces in the falling region.
+FPI is 0.0% at every scale — resolution loss goes entirely to TBI, never to a
+wrong name. The ceiling is **cross-view, not resolution**: everyone matches
+themselves within a recording (0.55–0.85) and collapses across two (0.14–0.45),
+and only the subject with frontal *gallery* references identified reliably — so
+the lever is gallery pose coverage (`docs/pose-expansion.md`), not a better
+landmark model. Averaging SCRFD's NMS-discarded landmark estimates lifts
+cross-clip TPI 41% → 49% for one forward pass.
+
+**Open.** Four identities and one shoot, so the shape is the result and the
+absolute rates are not. Both clips hold all four people, so there is no
+out-of-gallery class and the 10×-weighted out-of-cast misID is untested — holding
+one identity out of the gallery would fix that.
+
+## VR-014 — Audio-signature offset recovery
+
+**Depends on:** `sae_audio` exposing `compute_signature()` and
+`signature_from_mono()` — it drives the shipped C++, as VR-013 does, so the
+thing measured is the thing that ships.
+
+`scripts/validation/test_audio_offset.py` over
+`tests/fixtures/audio/bali_offset_200s.flac`: 200 s of public-domain film audio
+(the same Road to Bali clips the replay fixtures use), long enough for a 120 s
+window to slide past the ±600-frame search cap. The slide itself is numpy here
+on purpose — matching belongs to the consumer, so writing it out keeps this a
+test of the signature rather than of somebody's matcher.
+
+**Findings.** Alignment is a solved problem here: the offset is the nearest frame
+in every in-cap trial, worst error **46 ms against a 500 ms budget**, and 46 ms is
+the quantisation floor — offsets are whole 92.88 ms frames, so no correct answer
+can be worse. The `runtime/2` anchor's factor of two holds through real trimmed
+files, and out-of-cap offsets and unrelated content are both declined.
+
+**The score is where the slack is, and it costs a tier rather than accuracy.** It
+tracks sub-frame misalignment — 0.94–0.99 near a frame boundary, 0.69–0.73 at
+half a frame — so two thirds of correct alignments miss the server's 0.85 `audio`
+threshold and land in `loose`. UT-108 measures the fix rather than proposing one:
+±1 frame of slack in the score returns all 40 to `audio` (min 0.906) with false
+matches unmoved at 0.12–0.16, costing 81 ms of the budget. See
+[`SPEC.md`](SPEC.md) IR-004 — the score is normative in the server spec, so the
+change is theirs to make.
+
+**Open.** One source, one language, one era of recording. The shape (offset exact,
+score set by sub-frame phase) should hold generally, but the absolute scores are
+this fixture's.
 
 ## VR-001 — Dump audit