bench: search harness — HNSW recall vs speed, end-to-end hybrid_search latency

New clawhdf5-bench binary `search_harness`, the measurement baseline for the
search hot-path work. On deterministic clustered 384-dim data it reports HNSW
build time and, per ef, recall@10 against an exact scan, QPS and p50/p99; and
for HDF5Memory: ingest, checkpoint, open, first-query-after-open and
steady-state hybrid_search latency at 1K/10K (and 100K with --full). Optional
JSON output for tracking.

Baseline recorded in BENCHMARKS.md. It shows two problems: HNSW recall@10 does
not respond to ef and falls from 0.87 (1K) to 0.31 (100K) on clustered data,
and end-to-end hybrid_search is ~1000x slower than its vector stage because
every query rebuilds BM25 and rewrites the .h5 file.

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

crates/clawhdf5-bench/Cargo.toml

@@ -13,6 +13,10 @@ path = "src/bin/longmemeval_bench.rs"
 name = "memory_arena"
 path = "src/bin/memory_arena.rs"
 
+[[bin]]
+name = "search_harness"
+path = "src/bin/search_harness.rs"
+
 [[bin]]
 name = "footprint_bench"
 path = "src/bin/footprint_bench.rs"
@@ -48,6 +52,7 @@ harness = false
 
 [dependencies]
 clawhdf5-agent = { path = "../clawhdf5-agent" }
+clawhdf5-ann = { path = "../clawhdf5-ann" }
 clawhdf5-io = { path = "../clawhdf5-io" }
 mpi = { version = "0.8", optional = true }
 serde = { workspace = true }