docs(VR-012): the quality knee, and what it retires

Sharpness is not a sufficient statistic for identity loss. Six cells at
effectively identical measured sharpness span 15.3% to 91.0% TPI,
ordered entirely by source size, because a scalar keyed on
high-frequency energy cannot separate attenuated high frequencies from
destroyed spatial sampling. AR-028's "kept separate, not collapsed into
one scalar" now rests on a measurement rather than an argument -- and
the reasoning it used to rest on, that the aligned crop is
scale-normalised so a measure there cannot re-measure size, was wrong
and is corrected in place.

Variance of Laplacian -- the most widely used blur metric there is -- is
anti-predictive at fixed degradation on all three blur families. The
decile it calls sharpest is 2.6x less identifiable than the decile it
calls blurriest, monotone across ten bins, because within a cell its
residual variance is native contrast rather than detail, and hard
shadows and JPEG ringing raise it while making a face harder to match.
Gating on it would preferentially discard the more identifiable faces.

Blur breaks confidence, not identity: rank-1 holds at 80.2% where TPI is
15.3%, and FPI never left 0.1% in any of the 108 cells. Degradation
produces abstention, never a wrong name. That is also why sharpness
fails as a compute gate -- even a visually destroyed face stays 46.9%
identifiable, so a gate discards recoverable evidence at three times the
cost of the free size filter. Discount, do not gate; the rule AR-028
already stated now has evidence for why it is right rather than merely
cautious.

Records the shape a discount must have (flat, then a cliff between sigma
2 and 3), that its cost scales with proximity to the decision boundary
rather than with blur, and that the pose half of VR-012 has not been
run.

TRACES: VR-012, AR-028, AR-029, AR-030 | SR-002

mkdocs.yml

@@ -39,6 +39,7 @@ nav:
     - Best Model: best-model.md
     - Gallery Scope (Full vs. Limited): gallery-scope.md
     - Pose Expansion: pose-expansion.md
+    - Quality Knee (Blur and Size): quality-knee.md
     - LVFace Deep Dive: lvface-deep-dive.md
   - Full Experiment Log: model-bakeoff.md
   - Service Conversion (proposal): service-conversion.md