feat: integrate HNSW into agent search, fix Python 3.14 build

Resolves two gaps found in a project-state review:

1. Python build was broken: PyO3/numpy 0.23 caps at Python 3.13 but the
   environment has 3.14. Bumped to 0.28 and updated the two breaking APIs
   (PyObject -> Py<PyAny>, allow_threads -> detach). The extension module now
   imports and round-trips under Python 3.14, unblocking cargo build --workspace.

2. The "HNSW vector search over agent memories" headline was unwired:
   clawhdf5-ann had zero dependents and the agent used a linear cosine+BM25 scan.
   - clawhdf5-ann is now a live index: insert, mark_deleted (soft delete with a
     deleted bitset, traversed but never returned), compact, and a format
     version tag (v2) with backward-compatible load of v1 files.
   - clawhdf5-agent wires HNSW behind the `hnsw` feature (ON by default). The
     index mirrors the cache (node id == cache index) and self-heals: it rebuilds
     whenever hnsw_synced_len drifts from cache.len(), so unhooked pushes can't
     desync it. Non-indexable stores (no/zero-dim/mixed embeddings) and queries
     whose dim doesn't match fall back to the exact linear scan.
   - hybrid.rs gains merge_vector_keyword, shared by the linear and HNSW paths.
   - tests/hnsw_integration.rs validates recall vs a brute-force oracle plus
     insert/delete/batch behaviour.

Disable HNSW for exact search with `--no-default-features --features float16`.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

crates/clawhdf5-py/src/group.rs

@@ -57,7 +57,7 @@ impl PyGroup {
 #[pymethods]
 impl PyGroup {
     /// Get a child object (dataset or subgroup) by name or path.
-    fn __getitem__(&self, py: Python<'_>, key: &str) -> PyResult<PyObject> {
+    fn __getitem__(&self, py: Python<'_>, key: &str) -> PyResult<Py<PyAny>> {
         match &self.inner {
             GroupInner::Read { file, path } => {
                 let full_path = if path.is_empty() {
@@ -96,7 +96,7 @@ impl PyGroup {
     }
 
     /// List the names of all children (datasets and subgroups).
-    fn keys(&self, py: Python<'_>) -> PyResult<PyObject> {
+    fn keys(&self, py: Python<'_>) -> PyResult<Py<PyAny>> {
         match &self.inner {
             GroupInner::Read { file, path } => {
                 let group = if path.is_empty() {