Slice B: visual fidelity foundation — render-any-state Skia, parity tests, node visual properties, legacy mirror retirement

[ScottMorris]

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

What you author is what the disc shows; richer still imagery; single authored model.

• Skia renders any button state (normal/focus/activate) including shadows/glows (ButtonShadowType is currently authored + DOM-previewed but never drawn by skia/scene.rs)
• Group node rendering in both renderers (position offset + z-order container)
• Authoring/interaction extraction recursively flattens grouped buttons with accumulated offsets — AuthorableMenuRef::buttons() only scans top-level scene.nodes today, so nested buttons would render without spumux rectangles/navigation
• Shared visual properties on positioned nodes — opacity, rotation, corner radius (shape/image), 2-stop linear gradient fill, image fit/crop — implemented in both renderers in the same PR
• Renderer-parity golden tests (render-preview export vs canvas snapshot) wired into CI
• SceneNode::Video gains width/height; poster-frame rendering in both renderers
• Retire the legacy Menu.buttons mirror: migrate planner/validation/compiler-fallback readers onto authored_document, keep the one-time migrate_to_document lift on load, delete the write-back sync in project-store.ts; move legacy-only fields onto MenuDocument (notably motion_audio_asset_id, which resolved_motion_audio_asset_id() reads only from the legacy Menu)

Intersects #28, #29, #53, #55, #63, #64.