Add multi-valued genres field

[Copilot]

Description

Replaces singular genre field with multi-valued genres field to eliminate synchronization issues between singular/plural fields that caused repeated retagging operations.

Key changes:

• Database migration infrastructure: New migrations table tracks completed migrations. genre → genres migration runs once on first invocation, splitting on common separators (, , ; , /) with precedence order - only the first matching separator is used per genre string.

• Metadata source updates: MusicBrainz, Beatport, Discogs, and LastGenre plugins now populate genres list field instead of singular genre string.

• Template function enhancement: %first template function now splits on \x00 (multi-value delimiter) by default for seamless genre list handling.

• Deprecation handling: Metadata sources passing deprecated genre parameter automatically split on known separators with deprecation warning.

• Backward compatibility: Old genre database column remains intact but ignored, allowing rollback to older beets versions without data loss.

Migration behavior:

• Only migrates rows where genre is non-empty and genres is empty
• Uses SQL for performance (~2s vs 11min for 9000 tracks)
• Resumes if interrupted (CTRL-C safe)

Separator precedence:
When splitting existing genre strings, the first matching separator determines the split:

• "Rock; Pop, Jazz" → ["Rock", " Pop, Jazz"] (splits on ; only)
• "Rock - Alt, Indie" → ["Rock", "Alt, Indie"] (splits on - only)

This ensures deterministic migration but means mixed-separator genres may need manual cleanup post-migration.

To Do

• Documentation.
• Changelog.
• Tests.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.