Add option to write DR14 values to audio file metadata

[nixternal]

Add ability to write calculated DR values back to audio file tags/metadata. This would allow music players and library managers to display DR values alongside other metadata (artist, album, etc.).

Use case: Users want to see DR values in their music player without re-analyzing files each time. Writing to tags makes the information persistent and portable.

Implementation notes:

• Use mutagen library (already a dependency)
• Add CLI flag: --tag or --write-tags
• Write to appropriate tag fields:
  • DYNAMIC RANGE or DR custom tag
  • REPLAYGAIN_ALBUM_RANGE (existing standard)
• Support major formats: FLAC, MP3, M4A, OGG
• Should be opt-in (don't modify files by default)
• Add --dry-run option to preview what would be written

Acceptance criteria:

• --tag flag writes DR to file metadata
• Works for FLAC, MP3, M4A, OGG formats
• Doesn't crash or corrupt files
• Respects existing tags (doesn't overwrite unrelated metadata)
• Documentation updated with examples
• Tests for tag writing and reading
• Warning/confirmation before modifying files

Security considerations:

• Always work on copies or have clear warnings
• Validate file integrity after writing
• Handle read-only files gracefully