Starting a music playlist, album, or artist on shuffle always opened on
the list's first track: MusicQueueController.load anchored _order[cursor]
and shuffled only the rest, and _startQueue collapsed "no start track"
into startIndex 0, so the anchor was always the head.
Anchoring is right for the two callers that do have a track which must
play first -- the now-playing shuffle toggle, and a load with an explicit
start track -- so make "no explicit start" representable instead of
inferring it from the index: load takes int? startIndex and shuffles the
whole list, head included, when it is null. A start track the list turns
out not to contain now drops the anchor rather than falling back to 0.
Video playback was never affected: Plex shuffles server-side via
/playQueues and Jellyfin already shuffles its full local list.
The queue's Random is injectable so the service-level regression is
deterministic without depending on the SDK's seeded-PRNG sequence.
Close#1811
edde746
f63d0fe49e