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>

scripts/optimizer/reembed_gallery.py

@@ -27,8 +27,9 @@ from pathlib import Path
 
 REPO = Path(__file__).resolve().parent.parent.parent
 sys.path.insert(0, str(REPO / "scripts"))
-from sae_embed_loader import load_embedder  # noqa: E402
-from sae_gallery import load_gallery_hdf5, save_gallery_hdf5  # noqa: E402
+from sae_embed_loader import load_embedder, resolve_arcface  # noqa: E402
+from sae_gallery import (embedder_stamp, load_gallery_hdf5,  # noqa: E402
+                         save_gallery_hdf5)
 
 
 def find_dir(images_root: Path, jellyfin_id: str, name: str) -> Path | None:
@@ -58,6 +59,11 @@ def main():
     ref = load_gallery_hdf5(Path(args.ref))
     images_root = Path(args.images)
     embedder = load_embedder(args.build_dir, args.models_dir, args.arcface)
+    # TRACES: GR-004 | SR-001 — this script exists to produce a gallery in a
+    # DIFFERENT model's space from the reference. The output must therefore never
+    # inherit the reference's stamp; it carries the stamp of --arcface, which is
+    # the whole point of the bake-off being safe to run.
+    stamp = embedder_stamp(resolve_arcface(args.models_dir, args.arcface))
 
     out_actors = []
     n_ok = n_nodir = n_noemb = 0
@@ -84,7 +90,7 @@ def main():
             print(f"  [{i}/{total}] ok={n_ok} no_dir={n_nodir} no_emb={n_noemb}",
                   file=sys.stderr)
 
-    save_gallery_hdf5({"actors": out_actors}, Path(args.out))
+    save_gallery_hdf5({"actors": out_actors}, Path(args.out), stamp)
     n_emb = sum(len(a["embeddings"]) for a in out_actors)
     print(f"[reembed] {Path(args.arcface).stem}: {n_ok}/{total} actors, {n_emb} embeddings "
           f"→ {args.out}", file=sys.stderr)