feat: bind galleries to the embedder that built them

GR-004 — a gallery built with one embedding model is meaningless with another.
Cosine similarities across models are garbage but look entirely plausible, so
this fails silently and expensively; every measurement taken against a
mismatched pair would have been quietly wrong.

The stamp is the model basename plus a SHA-256 of its bytes, with embed_dim as
a cheap extra guard. The hash decides and the name explains, because neither
works alone: a name is a promise rather than a fact — models get re-exported in
place under an unchanged filename, which is exactly the case where the weights
differ and nothing else does — while a bare hash mismatch tells an operator
nothing actionable.

Mismatch is fatal in every mode with no bypass. Unstamped only warns, because
unstamped is unknown rather than known-bad, and an error firing on every legacy
gallery trains people to reach for the bypass reflexively. scripts/stamp_gallery.py
binds an existing gallery in place with no re-embedding, so the warning is a
migration step rather than a permanent state; --require-gallery-stamp promotes
it to an error once a site has migrated.

Two gaps found that would have defeated the requirement outright:

- Embedding dumps carried no stamp, so a replay — which has no live embedder —
  had nothing to check the gallery against. Dumps now carry embedder_model and
  embedder_sha256 as root attributes. Additive; schema_version stays 1. This is
  the same gap the dump audit identified independently.
- --merge produced one file holding two embedding spaces, which no later check
  can untangle. Merge paths now verify before writing.

The stamp also survives identity_matcher's calibration write-back, which would
otherwise have stripped it on the first analysis run — the check would have
worked exactly once.

Conflicts resolved additively: both branches appended a source to sae_gallery
and to the test target, and both edited the GR-004 register row.

Merged suite: 64 cases, 3199 assertions, passing on CPU with no GPU.

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

TRACES: GR-004, VR-001 | SR-001

docs/SPEC.md

@@ -1083,8 +1083,57 @@ surfaced as a build report.
   different models are meaningless but *look* plausible — this fails silently and
   expensively otherwise.
 
-**Gap:** named as step 4 of the `service-conversion.md` implementation plan;
-unbuilt. This is the highest-value small fix in the document.
+### The stamp
+
+Two fields, written together: the model file's **basename** and the **SHA-256 of
+its bytes** (plus `embed_dim` as a cheap extra guard). Stored as the `/embedder`
+group in the gallery HDF5, and as an optional top-level `"embedder"` object in
+the legacy JSON format.
+
+The hash *decides*; the name is what a human *reads*. Neither alone is enough. A
+name is a promise rather than a fact — models get re-exported, re-quantised and
+overwritten in place under an unchanged filename, which is exactly the case where
+the weights differ and nothing else does, so a name-only stamp is blind to the
+failure it exists to catch. A hash alone is correct but unactionable: *"expected
+3f2a…, got 9c1b…"* tells an operator nothing about what to do next. SHA-256 over
+the file is derived from the artefact rather than asserted about it, needs no
+registry kept up to date, and costs ~0.1 s for a 250 MB ONNX once per process.
+
+### Verdicts
+
+| Verdict | When | Default | Under strict mode |
+|---|---|---|---|
+| `match` | hashes agree | proceed | proceed |
+| `weak_match` | names agree, one side unhashable | **warn** | **error** |
+| `unstamped` | gallery predates GR-004 | **warn** | **error** |
+| `unknown_embedder` | gallery stamped, embedder unidentifiable | **warn** | **error** |
+| `mismatch` | proven different models | **error** | **error** |
+
+**A mismatch is fatal in every mode, with no bypass**, and the message names both
+sides — what the gallery was built with and what is loaded.
+
+The three "cannot prove it" verdicts warn loudly instead, because they describe an
+*unknown* state rather than a *known-bad* one, and because every gallery built
+before this requirement is unstamped. Hard-failing all of them would make the
+check something people route around rather than trust. Strict mode
+(`--require-gallery-stamp`, or `SAE_REQUIRE_GALLERY_STAMP=1`, which propagates to
+subprocesses) promotes them to errors — that is the mode measurement work runs in.
+`scripts/stamp_gallery.py` re-binds an existing gallery without re-embedding, so
+migration costs one command; that is what makes "warn" a temporary state rather
+than a permanent one.
+
+### Scope of the check
+
+Embedding **dumps** carry the same stamp (`embedder_model` / `embedder_sha256`
+root attributes, `scripts/optimizer/SCHEMA.md`): a replay has no live embedder, so
+the dump *is* the embedder as far as the gallery is concerned. Derived galleries
+(filter, cast-restrict) inherit their source's stamp; `--merge` and the JSON
+gallery merge check *before* writing, since a merged file holding two embedding
+spaces cannot be untangled afterwards by any later check.
+
+**Gap:** none. Stamped in `gallery_builder.cpp` and the Python builders; verified
+in `scene_analyze`, `scene_preview`, the `sae_kpn` matcher binding, `replay.py`,
+`optimize.py`, `movienet_eval.py` and the merge paths.
 
 ## GR-006 … GR-009 — Provenance tiers and poisoning guard