feat: output metadata tagging (--set-title/--set-artist/--set-album/--set-comment)

[seonghobae]

Summary

Adds output metadata tagging: four new CLI options — --set-title, --set-artist, --set-album, --set-comment — inject -metadata key=value pairs into the generated ffmpeg commands, so archived FLAC/Opus outputs carry meaningful tags that are searchable in players and music libraries.

Design

• _metadata_args(tags) helper produces ["-metadata", "key=value", ...] in sorted key order, so generated commands are deterministic. Values are passed through verbatim as single argv items (plans run without a shell), so spaces, quotes, $(...), and non-ASCII are all safe as given.
• ffmpeg semantics documented and relied upon: both plans already copy source metadata via -map_metadata 0. The -metadata pairs are injected after it (after -map_chapters 0, before -vn), so each provided key overrides that specific tag copied from the source while every other source tag is preserved.
• Threaded through the same chain as prefer_flac: parse_args -> _execute_conversions -> convert_file -> _convert_segment -> _execute_segment_conversion -> build_audio_plan / build_opus_plan, including the FLAC-too-large -> Opus fallback path, so segmented and fallback outputs are tagged identically.
• parse_args collects only the options actually provided into args.tags (empty dict when none passed).
• Byte-identical default: with no --set-* options, the generated ffmpeg argument lists are unchanged from before this PR (asserted by tests).

Tests

• parse_args tag collection: all four, partial subset, and default (empty tags, all set_* None).
• -metadata pair presence, ordering, and placement (after -map_metadata, before -vn) in both the FLAC and Opus plans.
• No-tags plans byte-identical to baseline plans (tags omitted vs None vs {}).
• Special characters in values pass through as single argv items.
• _execute_conversions forwards args.tags to convert_file (and None when empty).

Gates

• coverage run -m unittest discover -s tests + coverage report --include=media_shrinker.py: 100%
• interrogate -c pyproject.toml media_shrinker.py: 100% (PASSED)
• unittest discover -s tests: 125 tests, no new failures — exactly the 5 pre-existing macOS os.listxattr mock errors.

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