dual-source-queue-sync

A UI-facing track list and the playback engine's timeline both hold the queue, so the UI list is re-derived from the engine timeline by media id after every engine-side change, refused when the sizes disagree, and mutated on both sides for user reorders. Use when the queue on screen plays in a different order than it shows, when shuffle scrambles the list but not playback, or before adding a second place that writes…