Multi-value tags fan out

When a track's Album Artist (or Genre) holds more than one value, the track appears under each value separately, instead of inventing one fused entry.

Worked example. A track's Album Artist holds two values: yaiol; Ars Ricercata. That track appears under both yaiol and Ars Ricercata — once in each artist's branch — rather than under a single fake "yaiol; Ars Ricercata" artist. Multi-value artist tags fan out the same way multi-genre tags do.

Like sort-name grouping, this is automatic and tag-driven.