Add pathlib symlink and metadata OS calls

[chaliy]

What changed

This adds the pathlib symlink and metadata filesystem operations requested in issue #335.

• add core OS-call support for Path.readlink()
• add core OS-call support for Path.symlink_to(...)
• add core OS-call support for Path.chmod(...)
• add core OS-call support for Path.lstat() and Path.stat(follow_symlinks=False)
• extend the Python AbstractOS / OSAccess surface accordingly
• add Rust and Python coverage for the new calls

Why

Monty already exposed parts of pathlib filesystem access, but it could not round-trip symlink targets, create symlinks, change mode bits, or request non-following stat metadata. This closes that gap for the OS-call surface and the Python callback API.

Notes

• direct/read-write mount handling supports the new operations
• AbstractOS remains backward-compatible via the path_lstat() fallback to path_stat()
• OverlayMemory still reports Path.chmod() and Path.symlink_to() as unsupported; that mode needs an explicit in-memory symlink model for full parity

Validation

• make format-rs
• make lint-rs
• make lint-py
• cargo test -p monty --test os_tests --features ref-count-panic
• cargo test -p monty --test fs --features ref-count-panic
• `uv run pytest crates/monty-python/tests/test_os_calls.py crates/monty-python/tests/test_os_access.py crates/monty-python/tests/test_os_access_raw.py

[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks
⏩ 15 skipped benchmarks¹

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_{Comparing chaliy:codex/issue-335-pathlib-os-calls (babf080) with main (a8645d8)}

Footnotes

1. 15 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codecov]

Codecov Report

❌ Patch coverage is 91.48936% with 32 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/monty/src/fs/common.rs	88.23%	11 Missing and 7 partials ⚠️
crates/monty/src/fs/direct.rs	92.23%	0 Missing and 8 partials ⚠️
crates/monty/src/fs/dispatch.rs	95.65%	1 Missing and 1 partial ⚠️
crates/monty/src/fs/overlay.rs	95.55%	0 Missing and 2 partials ⚠️
crates/monty/src/fs/overlay_state.rs	0.00%	0 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[chaliy]

PR touches the monty-dataset, which triggers a considerable number of code paths that are not covered by unit tests. Should I still attempt to satisfy codecov?

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.