- Change FileInfoCache.mu from sync.Mutex to sync.RWMutex
- Get method uses RLock for concurrent read access
- Stats method uses RLock for read-only operation
- Double-check pattern for lock upgrades (TTL expiry, LRU move)
This improves concurrent read performance for the FileInfo cache,
which is read-heavy in static file serving scenarios.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>