derive-seek-from-progress-flow

Detect a scrub as a playhead that moved further than wall-clock time can account for, from the progress stream every platform already emits, because the platform's own discontinuity callback exists on one backend only and neither the item stream nor the transport stream fires when a scrubber is dragged. Use when a seek by one member of a shared session is never sent, or when a cross-platform layer needs an event…