fix(io): Add robust symlink cleanup to prevent resource leakage

[yurekami]

Summary

• Adds context manager support (__enter__/__exit__) to iovec and ioring classes for Pythonic resource management
• Adds atexit handler to clean up orphaned symlinks on normal process exit
• Adds global thread-safe symlink registry to track active symlinks
• Adds explicit close() methods with idempotent behavior for manual cleanup

Related Issue

Fixes #334 - Shared memory leakage due to symlinks not being properly deleted

Changes

The iovec class creates symlinks in /3fs-virt/iovs/ that point to shared memory. Previously, cleanup only happened in __del__, which is unreliable in Python (not guaranteed to run, timing issues, exception handling problems).

This PR improves cleanup reliability by:

1. Context manager support: Users can now use with make_iovec(...) as iov: for automatic cleanup
2. Atexit handler: Registered at module load to clean up any remaining symlinks on exit
3. Global registry: Thread-safe tracking of active symlinks for cleanup coordination
4. Explicit close(): Idempotent method that can be called safely multiple times

Example usage:

# Preferred: using context manager (new)
with make_iovec(shm, mount_point) as iov:
    ior.prepare(iov[:], True, fd, offset)
    # ...
# symlink automatically cleaned up

# Alternative: manual management
iov = make_iovec(shm, mount_point)
try:
    # use iov
finally:
    iov.close()  # explicit cleanup

Test plan

• Verify symlinks are cleaned up when using context manager
• Verify symlinks are cleaned up on normal process exit via atexit
• Verify symlinks are cleaned up when exception occurs in context
• Verify thread-safety of global registry under concurrent access
• Verify backward compatibility with existing code using __del__

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