Three new settings in Paths & Organization:
- Artist Tag Separator: choose comma, semicolon, or slash between artists
- Write multi-value ARTISTS tag: each artist as separate tag value for
Navidrome/Jellyfin multi-artist linking (FLAC ARTISTS key, ID3 TPE1
multi-value, MP4 multi-entry)
- Move featured artists to title: keep only primary artist in ARTIST
tag, append others as (feat. ...) in track title
All opt-in with defaults matching current behavior. Raw artist list
stored on metadata dict for tag writers to access without re-parsing.
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