Simplify multi-genre implementation based on maintainer feedback (PR #6169).
Changes:
- Remove multi_value_genres and genre_separator config options
- Replace complex sync_genre_fields() with ensure_first_value('genre', 'genres')
- Update all plugins (Beatport, MusicBrainz, LastGenre) to always write genres as lists
- Add automatic migration for comma/semicolon/slash-separated genre strings
- Add 'beet migrate genres' command for explicit batch migration with --pretend flag
- Update all tests to reflect simplified approach (44 tests passing)
- Update documentation
Implementation aligns with maintainer vision of always using multi-value genres
internally with automatic backward-compatible sync to the genre field via
ensure_first_value(), eliminating configuration complexity.
Migration strategy avoids problems from #5540:
- Automatic lazy migration on item access (no reimport/mbsync needed)
- Optional batch migration command for user control
- No endless rewrite loops due to proper field synchronization
External Python packages interfacing beets may want to use an in-memory
beets library instance for testing beets-related code.
The `TestHelper` class is very helpful for this purpose.
Previously `TestHelper` was located in the `test/` directory.
Now it is part of `beets` itself (`beets.test.helper.TestHelper`) and
can be easily imported.
