docs(s31): close 1 review MAJOR — vector store recorded as an unwired authoring primitive

docs/extending.md

@@ -146,11 +146,20 @@ is opt-in (`--semantic-retrieval`) rather than the default. If you wire one in,
 `semretrieval` currently imports no network module at all, and a guard test asserts exactly that;
 a real client belongs behind the `Embedder` protocol in *your* module, not inside this one.
 
-Optional persistence: `save_vector_store(dir, verdicts, embedder)` / `load_vector_store(dir)`
-write a byte-deterministic `vectors.npy` + `vectors.jsonl` pair (sorted by verdict id, atomic
-replace). It is a **rebuildable cache, never authoritative** — a missing store loads as `None`
-and the caller degrades to structural ranking, while a row/line mismatch raises rather than
-silently mis-ranking. `*.npy` is gitignored.
+**Optional persistence — an unwired authoring primitive.** `save_vector_store(dir, verdicts,
+embedder)` / `load_vector_store(dir)` write and read a byte-deterministic `vectors.npy` +
+`vectors.jsonl` pair (sorted by verdict id, atomic replace). A missing store loads as `None`; a
+row/line mismatch raises rather than silently mis-ranking. `*.npy` is gitignored.
+
+They have **no caller in `src/`**, and ranking never reads a persisted matrix: `HybridRanker.rank`
+re-invokes the embedder for every candidate on every call, so a persisted store would be bypassed
+even if one existed. They are offered to extenders, on the same footing as `write_verdict`,
+`promote_verdict` and `build_mcp_server` — public primitives the core deliberately does not wire
+into `run_project`. Wiring the cache in only becomes worth anything once the ranker is changed to
+consult it, which is a redesign rather than a hookup.
+
+*Known limitation:* the empty-store branch hardcodes `EMBED_DIM`, so a third-party `Embedder` of a
+different dimension writes a shape-inconsistent empty store. Stated, not fixed.
 
 ## Bevisst ikke bygget (90 %-kuttlista)