perf: Tier 4c — optional rayon parallelism for HNSW prune_connections

Add a default-off `parallel` feature to clawhdf5-ann (rayon optional dep),
matching the convention already used in clawhdf5-format/clawhdf5-agent.
Gate prune_connections' per-neighbor distance computation on it — a pure
read-only map with no shared mutable state, sorted immediately after, so
swapping to rayon's par_iter is low-risk.

Deliberately not touching build_with_metric's outer insert loop per the
original plan: it has genuine cross-iteration data dependencies (graph
mutation, entry-point updates) and needs its own correctness-focused
design pass. The win here is likely small since neighbor lists are
bounded by m/m_max0 (typically small) — this is a low-risk completeness
item, not a headline perf change.

Verified identical results with default features and --features parallel
across the full HNSW test suite (23/23 both ways), including the
build+search end-to-end tests (build_small_index, search_accuracy_cosine,
incremental_insert_matches_batch_recall).

crates/clawhdf5-ann/Cargo.toml

@@ -12,3 +12,7 @@ categories = ["algorithms", "science"]
 [dependencies]
 clawhdf5-format = { path = "../clawhdf5-format", version = "2.1.0" }
 clawhdf5-io = { path = "../clawhdf5-io", version = "2.1.0" }
+rayon = { version = "1", optional = true }
+
+[features]
+parallel = ["rayon"]