Replace the TrackId-only IsActivePlayer gate with a LivePlayerTrack source that follows the playing track when it is the host track or shares the host release; single-track Mix/Session unchanged at parity.
Add GET api/track/{trackEntryKey}/waveform/high-res (+ proxy), ITrackDataService.GetTrackWaveform; rewire visualizer to resolve the current track's EntryKey and re-fetch on track change. Retire the client mix-waveform read path.