feat: compare directory diffs by file content instead of mtime

[pompos02]

Summary

This PR improves non-git directory comparison (:CodeDiff dir ...) by switching modified-file detection from metadata-based checks to content-based checks.
Previously, directory diff mode treated files as modified using size + mtime.
Now it compares file bytes directly (chunked reads), which makes results accurate as timestamps are unreliable.

What changed

• Removed mtime from directory scan metadata.
• Added absolute file paths in scan results so files can be opened directly for comparison.
• Updated modified detection logic to:
  • early-return on size mismatch
  • otherwise read both files in chunks and compare byte-for-byte
  • mark as modified if open/read fails on either side
• Updated README wording to document content-based directory diff behavior.

Implementation details

The comparison is implemented with libuv file APIs to keep it fast.

Benchmark

Ran a benchmark on the TensorFlow repository (HEAD vs a random branch). The benchmark was executed without using the git index, and the comparison contained 812 changed files.

• Time (mean ± σ): 3.205 s ± 0.043 s [User: 2.039 s, System: 1.164 s]
• Range (min … max): 3.152 s … 3.323 s
• Runs: 15