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.

docs/index.md

@@ -5,12 +5,30 @@ film or TV episode — built on [KPN++](https://gitea.tourolle.paris/dtourolle/K
 (a C++20 Kahn Process Network library) for the detect → track → match → scene
 pipeline, with a Jellyfin-integrated gallery and an X-Ray-validated optimizer.
 
-## Start here
+## Start here — four questions this bake-off answers
 
-- **[Rep4 model bake-off + threshold re-tune](rep4-optimizer-results.md)** — the
-  current experiment log: model comparison, DE threshold tuning, held-out
-  validation, and the visual failure-mode evidence (frozen-bbox ghost tracks).
-  This is where the shipped `src/config.hpp` defaults come from.
+- **[Which model is best?](best-model.md)** — calibration curves first
+  (discriminative power, independent of any threshold), then F1 on the actual
+  benchmark. LVFace-B Glint360K wins both.
+- **[Whole gallery vs. limited (cast-restricted) gallery](gallery-scope.md)** —
+  restricting the matcher to a film's credited cast is a clean win on every
+  axis (+3.3pp F1, less than a third the misIDs), but isn't a shipped runtime
+  feature yet.
+- **[Does pose expansion help?](pose-expansion.md)** — a real training-set
+  effect that didn't reproduce on 5 held-out films once two methodology bugs
+  were caught and fixed. An honest null result, not a forced narrative.
+- **[Deep dive: LVFace-B Glint360K](lvface-deep-dive.md)** — the winning
+  model's held-out generalization gap, its two real failure modes (frozen-bbox
+  "ghost tracks"), and one case where it correctly identified an actor that
+  the X-Ray ground truth itself failed to credit.
+
+## The full technical log
+
+- **[Rep4 model bake-off + threshold re-tune](rep4-optimizer-results.md)** —
+  the complete experiment log behind the four pages above: the ROCm teardown
+  deadlock root cause and fix, DE concurrency tuning, the full 16-combo
+  results table, and every caveat. This is where the shipped `src/config.hpp`
+  defaults come from.
 - **[Optimizer experiments (prior round)](optimizer-experiments.md)** — the
   earlier scene-union-metric tuning pass, superseded by the per-second metric
   used in rep4 but kept for the ground-truth/architecture background.