YantrikDB

v0.6.4 — read connection pool for concurrent recall

Adds Vec<Mutex> pool of additional read conns (default
4 via YANTRIKDB_READ_POOL env). Eliminates serialization on recall
paths that previously all queued through the single Mutex.
SQLite WAL handles concurrent readers natively; the pool just
gives them independent locks.

Server-side benchmark deltas (v0.8.9 server):

• 10-conc recall: 7×200/3×503 → 10/10 (p99 341ms → 143ms)
• 50-conc recall: 0×200/47×503/3-timeouts → 33×200/17×503/0 timeouts

Forget-immediately-hides invariant verified: 0 stale reads
sequential, 0 stale under race (5 readers per probe × 5 probes).