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.

.gitignore

@@ -30,9 +30,12 @@ trt_cache/
 # ORT pre-optimized model cache (generated on first run, provider-specific)
 ort_cache/
 
-# Gallery JSON files (generated)
+# Gallery files (generated — HDF5 only, see src/gallery/gallery_store.cpp).
+# Legacy JSON galleries from before that switch are also excluded.
 gallery.json
 gallery_*.json
+gallery.h5
+gallery_*.h5
 
 # Calibration cache (generated alongside a gallery, per-embedder)
 *.calib_cache.csv
@@ -61,6 +64,22 @@ eval/probe/
 # Local reference repos kept for inspiration (each has its own .git)
 inspiration/
 
+# experiments/ has its own nested .gitignore for HDF5 galleries/dumps/X-Ray
+# corpus (all pushed/pulled via scripts/artifacts/{push,pull}_artifacts.sh to
+# the Gitea generic package registry instead of committed).
+# Exception to the blanket *.json rule above: the committed placeholder for
+# experiments/file-lut.json (see experiments/.gitignore).
+!experiments/file-lut.template.json
+
+# Site build output (mkdocs build). Rendered site is deployed to a
+# gitea-pages branch, never committed to a working branch.
+site/
+docs_site/
+
+# Images staged into docs/ from the artifact registry at build time
+# (scripts/docs/build_site.sh) — not committed, pulled fresh on each build.
+docs/assets/
+
 # Python
 __pycache__/
 *.pyc