perf(ann): unit-vector dot product and a reusable visited set

- Cosine distance was 1 - dot/(|a||b|), re-deriving both norms on every
  evaluation in the innermost loop of build and search. The index now stores
  unit vectors (prepared at build, insert, graph load and HDF5 load; the query
  once per search) and uses 1 - dot. Zero vectors stay zero, giving distance 1
  as before. Returned distances are unchanged.
- search_layer allocated a HashSet of visited nodes per call. It is now an
  epoch-stamped u32 array in thread-local scratch, reused across calls, so
  search(&self) stays shareable between threads.
- clawhdf5-accel caches the detected SIMD backend in a OnceLock.

Recall is identical. Build 2.75 -> 1.89 s (10K), ~38 -> 21 s (100K); QPS at
ef=64 22.7K -> 39K (10K), 10.4K -> 14K (100K).

Co-Authored-By: Claude Fable 5.1 <[email protected]>

CHANGELOG.md

@@ -32,6 +32,15 @@
   replayed from the WAL join the loaded index incrementally; a replayed update
   or delete invalidates it. `snapshot()` copies it. Batch saves no longer force
   a full index rebuild.
+- `clawhdf5-ann`: faster HNSW build and search with identical recall. The
+  cosine metric stores unit vectors and compares them with a plain dot product
+  (it re-derived both norms on every distance evaluation), and the per-call
+  `HashSet` of visited nodes is a reusable epoch-stamped array. Build 2.75 ->
+  1.89 s at 10K and ~38 -> 21 s at 100K; QPS at `ef = 64` 22.7K -> 39K at 10K.
+  Distances returned by `search` are unchanged (1 - cosine). Indexes loaded
+  from older HDF5 files are normalised on load.
+- `clawhdf5-accel`: the SIMD backend is detected once per process instead of
+  on every kernel call.
 - `clawhdf5-ann`: `HnswIndex::graph_to_bytes` / `from_graph_bytes` — graph-only
   serialization (checksummed, every neighbour id and level validated on load).
 - `clawhdf5-agent`: BM25 results are deterministic (ties break by record id),