feat(agent): HDF5Memory::search with source filters, re-ranking, confidence

`HDF5Memory::search(query_embedding, query_text, &SearchOptions)` is the
store's full search path. `SearchOptions::new(k)` is plain hybrid search
with the tuned default fusion; each further stage is opt-in:

- `with_sources([..])`: only records from these source channels. The
  filter applies before ranking, so a filtered search still returns up
  to k results, normalised over what it can return. The HNSW pool is
  over-fetched in proportion to what the filter removes, and the allowed
  records are scanned exactly whenever that costs fewer distance
  evaluations than the index would (~pool x M) — and as the fallback if
  the pool comes back short. Keyword matches are filtered too.
- `with_rerank(ReRankConfig)` re-ranks a max(3k, 10) candidate pool by
  relevance, recency, source authority and activation;
  `with_confidence(ConfidenceConfig)` drops low-confidence results;
  `at_time(now)` pins the recency clock.

These were reachable only through the OpenClaw backend, which is now
`search` with both on. Its Hebbian boost now goes to the k results it
returns rather than the whole 3k candidate pool. `hybrid_search` and
`hybrid_search_with` are wrappers and unchanged (tested bit for bit).

Measured on tank (search_harness --options-study --full, 3 runs): at
100K every filter — 50%, 10%, 1% of the store, and records far from the
query — returns the exact filtered top 10, and none is slower than an
unfiltered search (1%: 2.3 ms vs 4.6 ms). Re-rank + confidence costs
about 3%. A first version decided between index and exact scan by pool
size vs store size; it measured 0.976 recall at 12.3 ms on the
far-from-query filter, which is why the rule compares costs instead.

Tests: tests/search_options.rs (filter correctness and full pages via
both paths, far-from-query fallback, edge cases, equality with
hybrid_search_with, re-rank recency, confidence, boost scope).

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CLAUDE.md

@@ -97,6 +97,14 @@ Cargo workspace with 16 crates under `crates/` (plus `libaec-sys`, an internal F
   must open in h5py — `f32` datasets and empty datasets did not until
   2026-09-23 (see `docs/known-issues.md`); the agent's `h5py_interop` test
   guards a whole store.
+- `HDF5Memory::search(query_emb, text, &SearchOptions)` is the full search
+  path: optional source-channel filter (applied before ranking; exact scan of
+  the allowed records whenever cheaper than `pool × M` index distance
+  evaluations, and as the fallback when the pool comes back short), fusion,
+  activation scaling, optional re-ranking and confidence rejection.
+  `hybrid_search`/`hybrid_search_with` are thin wrappers; the OpenClaw
+  backend is `search` with re-rank + confidence on. Measure changes with
+  `search_harness --options-study`.
 - `MemoryConfig::compression` is off by default; when on, embeddings are
   deflate-compressed, or Zstd with the agent's `zstd` feature (links libzstd).
 - `Dataset::verify_provenance()` (clawhdf5 facade, `provenance` feature, on by