Slice E: motion thumbnails / video buttons

[ScottMorris]

Part of the rich menu editor epic. Plan: docs/rich-menu-editor-plan.md §4 Slice E. Blocked by Slice D.

The classic "moving chapter tile" idiom.

• Compile button videoAssetId and SceneNode::Video: ffmpeg overlay chains composite per-button/node video into the motion background at bounds (approach 2 in docs/motion-menus.md), beneath a standard highlight
• Editor: video asset pickers on buttons/video nodes, poster-frame display, scrub-on-hover
• Timeline scrubbing shows composited motion
• Chapter-grid generator: opt-in motion tiles sourced from the thumbnail/preview cache timestamps