fix(storage): recycle() respawns reader workers serially with retry

CI (macos-26) hit "malformed database schema (sqlite_master)" in the
reader-pool recycle test — the exact Darwin WAL/SHM bring-up race
start() already guards against by booting slots serially with per-slot
retry. recycle() still spawned all workers in parallel without retry,
reintroducing the race on slow runners. It now reuses startSlot().