Detach and stop the active playback engine before asynchronous scene teardown so switching to a non-playable scene cannot leave narration, timers, or effects running.\n\nRecheck engine ownership after lecture-session creation in manual and auto-play paths, close stale sessions, and reject progress callbacks from detached engines. Add regression coverage for all three races.
Co-authored-by: wyuc <wang-yc24@mails.tsinghua.edu.cn>
