perf(fs): speed up native metadata conversion

[hardfist]

Summary

• Add a Unix metadata conversion fast path in rspack_fs::FileMetadata using std::os::unix::fs::MetadataExt.
• Keep the existing SystemTime conversion path for non-Unix targets.

Performance Impact

Native filesystem metadata is queried by resolver, snapshot, and cache invalidation paths. This avoids repeated SystemTime construction and duration_since conversion for Unix mtime and atime values while preserving the existing millisecond timestamp semantics.

The expected benefit is lower CPU overhead in high-cardinality stat-heavy paths, especially during dependency resolution and persistent cache snapshot checks.

[github-actions]

📦 Binary Size-limit

Comparing 7f8a7b2 to fix(esm_library): preserve static source-phase external bindings (#14674) by Elecmonkey

🙈 Size remains the same at 66.74MB

[codspeed-hq]

Merging this PR will not alter performance

✅ 43 untouched benchmarks
⏩ 47 skipped benchmarks¹

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_{Comparing perf/fs-metadata-fast-path (7f8a7b2) with main (df598e4)}

Footnotes

1. 47 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. ↩