Adds nextTitles (array of up to 3 display strings) plus mirrored single
nextTitle/nextArtist/nextTrackTitle/nextFile for the immediate next
track. Drives the Music Box overlays' 3-up "coming next" lines without
overlays needing to query MPD themselves.
Reads MPD status.nextsong, walks playlistinfo for the next 3 indices,
applies the existing _flatten_tag + display-string cascade. Fields are
None / [] at end of queue. Backwards-compatible: overlays that don't
read the new keys are unaffected.
