perf(ann): batched bulk build, parallel with the parallel feature

Profiling the build showed 90% of all distance evaluations are in back-link
pruning (40.8M of 44.9M at 10K): every overflow re-runs the diversity
heuristic pairwise over ~max_conn candidates.

The bulk build now inserts in batches: plan every node's neighbours against
the graph as it stood when the batch began (read-only, so plans are
independent), link, then prune each overflowing list once. A node gaining
several back-links in a batch is pruned once rather than once per link, so
this is faster even single-threaded (10K: 1676 -> 1074 ms). With `parallel`,
planning and pruning use rayon (10K: 388 ms; 100K: ~21 s -> 5.9 s on 16
cores). Batches start at one node and are capped at 1/16 of the linked graph
and 512 nodes; a node that raises the top layer gets a batch to itself. The
result is deterministic and identical with or without the feature (one code
path; test compares two builds byte for byte).

Parallelising within a single insert was tried first: 1.45x on 16 cores, tasks
too small. Incremental insert() stays sequential.

Recall on clustered data is unchanged or slightly better; uniform random data
dips slightly (10K, ef=64: 0.474 -> 0.444).

clawhdf5-agent's `parallel` feature now passes through to the index.

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

CHANGELOG.md

@@ -3,6 +3,13 @@
 ## Unreleased
 
 ### Search
+- `clawhdf5-ann`: **faster index builds.** Back-link pruning is 90% of a
+  build's distance evaluations; the bulk build now inserts in batches and
+  prunes each overflowing neighbour list once per batch (10K: 1676 -> 1074 ms).
+  With the `parallel` feature, planning and pruning run on a thread pool (10K:
+  388 ms, 100K: ~21 s -> 5.9 s on 16 cores). The graph is deterministic and
+  identical with or without the feature. `clawhdf5-agent`'s `parallel` feature
+  now enables it for the agent's index.
 - `clawhdf5-ann`: `HnswIndex::search` returned fewer than `k` results — often
   none — when the records nearest the query had been deleted: it collected `ef`
   candidates, *then* dropped the deleted ones, *then* took `k`. Deleted nodes