chore(release): v2.4.0

Bump all workspace crates, the node package and pyproject to 2.4.0, finalize
the changelog and add upgrade notes for the search behaviour changes.

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

CHANGELOG.md

@@ -1,6 +1,24 @@
 # Changelog
 
-## Unreleased
+## v2.4.0 (2026-09-19)
+
+### Upgrade Notes
+- **Search results improve on upgrade.** The HNSW index now reaches true
+  neighbours it previously could not (recall@10 0.31 -> 0.98 at 100K records on
+  clustered data), so `hybrid_search` rankings change for the better. The agent
+  rebuilds its index from the store automatically; a standalone `HnswIndex`
+  persisted with `to_hdf5_bytes` keeps its old graph until rebuilt.
+- **`hybrid_search` no longer writes the store.** Hebbian activation boosts are
+  persisted by the next checkpoint (any flushing write, `flush_wal`, or when
+  the `HDF5Memory` is dropped) instead of inside every query; a crash before
+  then forgets only the boosts since the last checkpoint. Activation weights
+  are now capped at 16.
+- A new sidecar file, `<store>.h5.ann`, holds the vector index graph. It is
+  derived data: safe to delete (the index is rebuilt), copied by `snapshot()`,
+  and worth including when copying a store by hand to avoid a rebuild.
+- `BM25Index` no longer caches IDF and gained `add_document`,
+  `remove_document`, `pad_to`, `scores`, `len` and `is_empty`; results are now
+  deterministic (ties break by record id).
 
 ### Search
 - `clawhdf5-ann`: **HNSW recall fix.** Neighbours were chosen as the plain