The music library is rips and discography packs whose filenames say nothing the
matcher needs. The authority is Jellyfin's own audio rows: one per track, with
the album and album-artist ids it carries in BaseItemProviders (the item JSON
holds an empty ProviderIds for albums in this build).
Grouping is what the tags assert, not where files sit: (album-artist tag, album,
year), case-folded. A compilation is one album whose tracks name many performers,
so it files once under Various Artists; two album-artist tags on one album name
are two albums. The folder spelling is the dominant one, because a name taken
from whichever row sorted first depends on row order and re-names albums that are
already filed.
An occupied path is never a destination: the executor refuses occupied targets,
so a plan that aims at one fails after the renames meant to clear it. A file
standing on its own target, or already carrying a collision suffix, is a keep -
otherwise the plan shuffles instead of being a fixed point.
