docs: rep4 bake-off write-up, MkDocs site, artifact-registry-backed experiments

docs/rep4-optimizer-results.md is the main deliverable: the model bake-off +
threshold re-tune experiment log, including the ROCm teardown deadlock root
cause and fix, DE concurrency tuning, the 16-combo results table, held-out
validation against 5 films never seen by the optimizer (macro F1 67.4% vs.
75.3% training — a real generalization gap), the frozen-bbox "ghost track"
failure mode found via annotated frame evidence, calibration curves per model,
and an isolated-effects breakdown of gallery scope vs. pose expansion.

MkDocs site (mkdocs.yml, docs/index.md) renders docs/*.md; scripts/docs/
pulls referenced images from the artifact registry and generates the
calibration chart at build time (see the tooling commit) rather than
committing images to the repo.

experiments/ now keeps only scripts + README + SESSION_STATE.md in git — every
data artifact (galleries, dumps, X-Ray corpus, montage frames, trajectories,
manifests, results) moved to the Gitea package registry. film-lut.template.json
is the committed placeholder for the gitignored file-lut.json (real local
movie paths, never shared — some source filenames carry scene-release tags).

Adds models/transnetv2.onnx (via Git LFS, matching the other ONNX models) for
the new scene-detection path.

docs/index.md

@@ -0,0 +1,33 @@
+# scene-actor-extraction
+
+A face-recognition pipeline that finds when each actor appears on screen in a
+film or TV episode — built on [KPN++](https://gitea.tourolle.paris/dtourolle/KPN)
+(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
+
+- **[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.
+- **[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+  earlier scene-union-metric tuning pass, superseded by the per-second metric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.
+- **[Service conversion (proposal)](service-conversion.md)** — design sketch
+  for an idle-GPU Docker worker, not yet built.
+
+## Reproducing the benchmarks
+
+Gallery `.h5` files, embedding dumps, the X-Ray corpus, montage frame images,
+and DE trajectories are not committed to this repository — they're pushed to
+the Gitea package registry and pulled on demand:
+
+```bash
+scripts/artifacts/pull_artifacts.sh galleries
+scripts/artifacts/pull_artifacts.sh experiment-data
+scripts/artifacts/pull_artifacts.sh montage-frames <film-slug>
+```
+
+See `scripts/artifacts/push_artifacts.sh` for the upload side (requires a
+`GITEA_TOKEN` with package write scope).