make check/CI never exercised the race detector over this PR's
concurrency (evictor goroutine, write-pressure channel, the new
per-hash contentLock, concurrent store/evict). Minimal, scoped change:
add -race to the existing go test invocation. Full suite is clean
under it (make check passes; go test -race ./... completes in under
7s, well inside the 30s timeout).
