Keyed within-radius operator for vector distances in deployed rules (VV5b) #413

Part of epic #305. Split from #327 (VV5), whose top-k part keeps hnsw_nearest in deployed rules as a keyed view.

Problem. A persistent rule that keeps the candidates within a distance of each query vector (within_radius<R, Id, D> over a distance such as D = cosine(QV, V)) is not maintained: it is refused with inputlayer_views_pending{reason="vector"} and every read evaluates it, comparing every query vector with every candidate. Distance functions in deployed rules arrive with VV1 (#323).

Target. A keyed within-radius operator in the view maintainer, beside the top-k operator of #327 (src/view_maintainer/nearest.rs): per distinct query key the candidates within radius R, with their distances; on query-key insert or change one radius search of the index (exact below engine.vector_exact_rows); on candidate insert or vector change an exact distance check against every key (O(keys)); on candidate delete removal from the keys holding it, with no refill. Row-level deltas at the commit's revision; distances computed with the index's own measure, so a view and a read agree to the bit; semantics and event table in vectors.mdx next to the top-k one.

Acceptance. Oracle on random histories (query-key inserts/changes, candidate inserts/deletes/updates): exact-mode results equal recompute at every revision; the operator's state equals a fresh evaluation after every event; candidate insert with 10,000 keys <= 10 ms p50 and query-vector change <= 3 ms p50 at 1M x 384-dim on the benchmark host.

Size M. Dependencies VV1 (#323), #327 (top-k operator). Order slot 8.