docs: four focused findings pages (best model, gallery scope, expansion, deep dive)

Splits the rep4 write-up's key findings into their own linkable pages:
- best-model.md: calibration curves first (discriminative power, independent
  of any threshold), then F1 on the benchmark — LVFace-B Glint360K wins both.
- gallery-scope.md: whole vs. cast-restricted gallery, isolated from model and
  expansion choice — restriction wins on every axis, but isn't a shipped
  runtime feature yet.
- pose-expansion.md: the training-set expand_gallery effect, and the held-out
  replication attempt that found it doesn't reproduce (5 films, 2 models,
  after catching and fixing a replay-timeout truncation bug and a bbox
  first-match-instead-of-best-match bug in the comparison harness itself). An
  honest null result, with the methodology errors documented since they're
  exactly the kind that manufacture a false "it works!" finding.
- lvface-deep-dive.md: the winning model's held-out generalization gap, its
  two failure modes (frozen-bbox ghost tracks), and a verified case (cross-
  checked against Jellyfin's independent cast metadata) where LVFace
  correctly identified an actor that X-Ray's ground truth failed to credit.

Adds a "report-highlights" artifact-registry package (scripts/artifacts/
push_artifacts.sh, pull_artifacts.sh) for hand-picked illustrative frames that
aren't reproducible via the automated best/worst montage selection, and wires
pulling it into scripts/docs/build_site.sh.

mkdocs.yml

@@ -18,7 +18,12 @@ theme:
 
 nav:
   - Home: index.md
-  - Rep4 Bake-off & Re-tune: rep4-optimizer-results.md
+  - Findings:
+    - Best Model: best-model.md
+    - Gallery Scope (Full vs. Limited): gallery-scope.md
+    - Pose Expansion: pose-expansion.md
+    - LVFace Deep Dive: lvface-deep-dive.md
+  - Rep4 Bake-off & Re-tune (full log): rep4-optimizer-results.md
   - Optimizer Experiments (prior round): optimizer-experiments.md
   - Service Conversion (proposal): service-conversion.md