fix(identity): resolve race conditions in identity file generation

[vuneumaaa]

Summary

Fixes race conditions in identity file generation by implementing:

• Atomic file writes using tempfile + os.replace
• Advisory file locking (fcntl on Unix, msvcrt on Windows)
• Safe Windows locking with position preservation
• IdentityCorruptedError for corrupted identity detection
• AAIP_IDENTITY_PATH environment variable support

Motivation

Concurrent access to identity files could lead to:

• Corrupted files
• Lost identities
• Failed PoE verification

This fix ensures safe concurrent behavior across platforms.

Validation

• mypy: PASS
• pytest: PASS (38 tests)

Notes

• Minimal changes aligned with roadmap Issue Race Condition Identity #3
• Cross-platform behavior verified (Windows + Unix)

Closes #3