GR-004: bind galleries to the embedder that built them

A gallery is only valid for the embedder that produced its vectors. Cosine
similarities across models are meaningless but *look* plausible, so the mistake
is silent and every measurement taken afterwards is suspect. Stamp the embedder
identity into the gallery at build; verify it at every load.

The stamp is the model file's basename plus the SHA-256 of its bytes (plus
embed_dim). The hash decides, the name explains. A name alone is a promise
rather than a fact — models get re-exported and overwritten in place under an
unchanged filename, which is exactly the case where the weights differ and
nothing else does. A hash alone is correct but unactionable in an error message.
SHA-256 is derived from the artefact, needs no registry kept current, and costs
~0.1s for a 250MB ONNX, memoised per process.

Mismatch is a hard error in every mode, with no bypass, naming both sides.

Unstamped legacy galleries warn loudly and proceed: unknown is not known-bad,
and hard-failing every pre-existing gallery would turn the check into something
people disable rather than trust. --require-gallery-stamp (or
SAE_REQUIRE_GALLERY_STAMP=1, which propagates to subprocesses) promotes that to
a hard error — the mode measurement work should run in. scripts/stamp_gallery.py
re-binds an existing gallery with no re-embedding, so "warn" is a cheap state to
leave rather than a permanent one.

Embedding dumps carry the same stamp: a replay has no live embedder, so the dump
is the embedder as far as the gallery is concerned. Derived galleries inherit
their source's stamp; --merge and the JSON gallery merge check before writing,
since one file holding two embedding spaces cannot be untangled afterwards.

Verified in: scene_analyze, scene_preview, the sae_kpn matcher binding,
replay.py, optimize.py (once per film at startup, before the first evaluation),
movienet_eval.py and both merge paths.

Stamp logic lives in src/gallery/embedder_stamp.{hpp,cpp} and its Python twin
scripts/sae_stamp.py, kept dependency-light so replay subprocesses do not pay
sae_gallery's requests/Pillow import to ask whether two models match.

Tests: 12 new cases in test_gallery_store.cpp covering the comparison logic,
both round trips, and the SHA-256 vectors that guarantee the C++ and hashlib
stamps agree. No ONNX or GPU required.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/plan.md

@@ -260,13 +260,23 @@ Context crops opt-in behind `--dump-unidentified-crops`.
 
 # Gallery
 
-## GR-004 — Model binding
+## GR-004 — Model binding — **DONE**
 
-**Depends on:** nothing. **Startable immediately, highest value per line.**
+**Depended on:** nothing. Landed before any measurement work, as intended.
 
-Stamp embedder identity into the gallery at build; verify at load in
-`scene_analyze`, `replay.py` and the optimizer. Mismatch is a hard error naming
-both sides.
+Stamp = model basename + SHA-256 of the ONNX, written as the `/embedder` group at
+build time (`gallery_builder.cpp`, `sae_gallery.save_gallery_hdf5`) and verified
+at load in `scene_analyze`, `scene_preview`, the `sae_kpn` matcher binding,
+`replay.py`, `optimize.py` and `movienet_eval.py`. Mismatch is a hard error naming
+both sides, with no bypass. Embedding dumps carry the same stamp, since a replay
+has no live embedder to check against.
+
+Unstamped legacy galleries **warn loudly and proceed** rather than failing:
+unknown is not known-bad, and hard-failing every pre-existing gallery would turn
+the check into something people disable. `--require-gallery-stamp` /
+`SAE_REQUIRE_GALLERY_STAMP=1` promotes that to a hard error — measurement runs
+should set it. `scripts/stamp_gallery.py` re-binds an existing gallery without
+re-embedding, so the warning state is cheap to leave.
 
 Cross-model similarities are meaningless but *look* plausible — this fails
 silently and expensively, and it would corrupt every measurement taken during the