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

[tony]

Summary

• Add FileLock class for cross-platform file locking with context manager support
• Add atomic_init function for race-free initialization (useful with pytest-xdist)
• Support both sync and async usage patterns

Test plan

• Unit tests for FileLock (acquire, release, context manager, timeout)
• Tests for atomic_init with concurrent access simulation
• Cross-platform compatibility (uses fcntl on Unix)

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 63 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.56%. Comparing base (f0e705d) to head (bbb80be).

Files with missing lines	Patch %	Lines
src/libvcs/_internal/file_lock.py	69.90%	58 Missing and 4 partials ⚠️
tests/_internal/test_file_lock.py	99.57%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #501      +/-   ##
==========================================
+ Coverage   53.22%   55.56%   +2.34%     
==========================================
  Files          38       40       +2     
  Lines        5676     6117     +441     
  Branches     1063     1095      +32     
==========================================
+ Hits         3021     3399     +378     
- Misses       2144     2202      +58     
- Partials      511      516       +5     

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