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]>

crates/clawhdf5-accel/src/lib.rs

@@ -61,8 +61,14 @@ pub enum Backend {
     Scalar,
 }
 
-/// Detect the best available SIMD backend at runtime.
+/// The best available SIMD backend, detected once per process. Every kernel
+/// dispatches through this, so it sits in the innermost loop of every search.
 pub fn detect_backend() -> Backend {
+    static BACKEND: std::sync::OnceLock<Backend> = std::sync::OnceLock::new();
+    *BACKEND.get_or_init(detect_backend_uncached)
+}
+
+fn detect_backend_uncached() -> Backend {
     #[cfg(target_arch = "aarch64")]
     {
         return Backend::Neon; // Always available on aarch64