Test audio fixture guidance

The current unit tests avoid committing real audio binaries and instead use `byte[]` plus a mocked `AudioTagExtractor`.

Recommended real-file fixtures if you later want integration tests:

1. `valid-square-cover.flac`
   - Tags: `title`, `artist`, `album_artist`, `album`, `track=01/12`
   - Embedded PNG cover `320x320`
   - UTF-8 encoded text

2. `missing-cover.mp3`
   - Same core tags as above
   - No embedded artwork
   - Expected validator result: `COVER_MISSING`

3. `bad-track.m4a`
   - Track stored as `1` instead of `01/12`
   - Expected validator result: `INVALID_FORMAT`

4. `garbled-title.ogg`
   - Title intentionally stored with broken encoding bytes so it decodes to `Bad � Title`
   - Expected reader encoding result: `UNKNOWN`

To generate those fixtures quickly, ffmpeg plus a tagging tool is enough:

```bash
ffmpeg -f lavfi -i sine=frequency=440:duration=1 -c:a flac valid.flac
```

Then embed tags and artwork with a dedicated tag editor, or continue mocking `AudioTagExtractor` in unit tests for faster coverage.