feat(agent): build the vector index in parallel by default

`parallel` joins the agent's default features, so the HNSW bulk build uses the
thread pool: cold index build at 10K records 1152 -> ~380 ms in a same-moment
A/B (the graph is identical either way). Nothing else on the measured paths
changes — ingest, checkpoint, open and steady-state query times are the same
with the feature on or off. Adds rayon to the default dependency set; opt out
with `--no-default-features --features float16,hnsw`.

Harness: `--e2e-only` runs the end-to-end section without the index
benchmarks. Note for anyone comparing numbers: this machine's absolute timings
drifted ~1.5x over a long session, so only same-moment A/B runs are
comparable.

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

crates/clawhdf5-agent/Cargo.toml

@@ -45,7 +45,7 @@ name = "memory_bench"
 harness = false
 
 [features]
-default = ["float16", "hnsw"]
+default = ["float16", "hnsw", "parallel"]
 float16 = ["half"]
 # Rayon-parallel brute-force search strategies, and a parallel bulk build of
 # the HNSW index (same graph, several times faster on a multi-core machine).