perf(compile): avoid blocking fs reads in the async compile path for the package-config check #2358

Follow-up from the #2315 review (Rust reviewer, unposted theme).

package_config_check::collect_warnings runs on the async compile path but uses blocking std::fs (read_dir, metadata, read_to_string) to scan at most nine small files at the repository root.

The cost is negligible today, and compile does other synchronous I/O. Even so, the scan should either use tokio::fs or spawn_blocking, or document why blocking I/O is acceptable here, so the pattern doesn't spread to larger scans.

Acceptance criteria

• Either the scan runs off the async executor, or a comment justifies the bounded blocking I/O.
• No behaviour change, and the existing tests pass.