_internal(feat[file_lock]): Add typed, asyncio-friendly file locking module

why: Enable safe concurrent access to shared resources across multiple
processes, particularly for pytest-xdist parallel test execution.

what:
- Add FileLock class with context manager support
- Add atomic_init() for one-time initialization with locking
- Support both sync and async usage patterns
- Use fcntl for Unix file locking

Author: tony