Slice C: motion menu build backend + MenuCompiler seam

[ScottMorris]

Part of the rich menu editor epic. Plan: docs/rich-menu-editor-plan.md §4 Slice C; pipeline detail in docs/motion-menus.md. Blocked by Slice B.

Motion backgrounds and audio beds actually build; the plan-time block is removed.

• Motion pipeline: loop-segment extraction from loopStartSecs, scene PNG composited over looping video, AC-3 audio bed, intro cell + loop cell with <post> jump, loop-count/timeout via g-register post-commands
• Carve the MenuCompiler trait seam (render_states → compose_background → mux); ffmpeg/dvdauthor types stop appearing in shared build code
• Planner: replace the "switch these menus back to still mode" error (planner/helpers.rs) with real motion jobs
• Inspector: loop-point scrub preview of the background video (<video> + convertFileSrc)
• Validation rules from docs/motion-menus.md §Validation