fix: run the test suite with -race

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).

script/test

@@ -17,7 +17,7 @@ run_with_cgo_deps() {
 main() {
     cd "$ROOT"
     echo "Running tests..."
-    run_with_cgo_deps "CGO_ENABLED=1 go test -timeout 30s -v ./..."
+    run_with_cgo_deps "CGO_ENABLED=1 go test -timeout 30s -race -v ./..."
 }
 
 main "$@"