fix(core): key the embedding cache by the embedder's identity, not its name (#1536)

CHANGELOG.md

@@ -16,6 +16,8 @@
 
 ### Fixed
 
+- **The embedding cache no longer serves the vector of another model (#1536)** - the persistent cache (`embed_cache.db`) was keyed by the embedder's constant name (`openai`, `ollama`, `embeddinggemma-q4`), so after changing `[embedding] model`, `dims` or the endpoint, an already cached text came back with the old model's vector, and a vector produced by the cloud fallback was cached under the primary's name. The key is now an identity the embedder owns (backend, model, dimensions, and for `openai` and `ollama` a hash of the endpoint; the API key is not part of it), a cached vector whose length is not the embedder's dimensions is ignored, an empty vector is not cached, and the fallback wraps the cached primary so cloud vectors never enter the cache (with a fallback configured, a repeated text whose primary embedding fails now calls the cloud each time instead of being served from the cache). Rows written under the old key are deleted once when the cache is first opened (`PRAGMA user_version` 1); the first embeddings after the upgrade are recomputed. `Embedder` gains a provided method `identity()` (default `name:dims`), so existing implementations keep compiling.
+
 - **`UTEKE_HOME` pointing at another store no longer routes to a running server (#1445)** - with `[server] enabled = true` and `uteke-serve` running, `UTEKE_HOME=/tmp/iso uteke remember ...` was sent to the server and landed in the server's store instead of `/tmp/iso`. The server now reports which store it serves on the new `GET /store` (LAB route, token required when auth is on): `{"id": "<16 hex digits>"}`, a SHA-256 prefix of the store's canonical path, never the path itself. Before routing to a loopback server the CLI compares that id with the store it resolved and runs locally on a mismatch (with a warning on stderr). The probe sends `UTEKE_AUTH_TOKEN` like any routed command; without it an auth-enabled server refuses the command with 401, so nothing lands in the wrong store. A server without the route (404), an unreachable one or a non-loopback one keeps the previous routing. `docs/core-contract.json` gains `/store` in the `lab` list only.
 
 ### Changed