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>

src/main.cpp

@@ -45,6 +45,7 @@
 
 #include "config.hpp"
 #include "types.hpp"
+#include "gallery/embedder_stamp.hpp"
 #include "gallery/gallery_store.hpp"
 #include "nodes/frame_source_node.hpp"
 #include "nodes/camera_position_change_detector_node.hpp"
@@ -114,6 +115,7 @@ static Config parse_args(int argc, char** argv) {
         else if (arg("--detector"))        cfg.detector_model   = next();
         else if (arg("--detector-engine")) cfg.detector_engine  = next();
         else if (arg("--arcface"))         cfg.arcface_model    = next();
+        else if (arg("--require-gallery-stamp")) cfg.require_gallery_stamp = true;
         else if (arg("--arcface-engine"))  cfg.arcface_engine   = next();
         else if (arg("--conf"))            cfg.detector_conf    = std::stof(next());
         else if (arg("--max-faces"))       cfg.max_faces        = std::stoi(next());
@@ -167,6 +169,11 @@ int main(int argc, char** argv) {
     ActorGallery gallery;
     try {
         gallery = load_gallery(cfg.gallery_path);
+        /// TRACES: GR-004 | SR-001
+        // Hard startup error before a single frame is decoded: a gallery built
+        // with another embedder yields plausible-looking, meaningless matches.
+        verify_gallery_embedder(gallery, cfg.gallery_path, cfg.arcface_model,
+                                cfg.require_gallery_stamp);
     } catch (const std::exception& e) {
         std::cerr << "Gallery error: " << e.what() << "\n";
         return 1;