Replace the Delete+LoadOrStore pattern with CompareAndSwap to
atomically swap in a fresh mutex when a corrupted entry is detected.
Changes:
- Use a retry loop with CAS instead of Delete+LoadOrStore, avoiding
the race where two goroutines could create different mutexes
- Add nil check: a nil *sync.Mutex passes the type assertion but
panics on Lock() — now handled by the CAS recovery path
- Use continue-loop retry instead of unchecked second assertion
CompareAndSwap guarantees that when multiple goroutines detect a
corrupted entry, they converge on the same replacement mutex.
