Slice F: themes & components (Templates rail, HeroTitleButton, ChapterThumbnailTile)

[ScottMorris]

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

theme_ref and the Templates rail stop being stubs; generated menus restyle without regeneration.

• Theme document (typography set, ButtonStyleMap, highlight palette, spacing tokens, background treatment) + resolution order: explicit node override → component default → theme token → model default
• Templates rail lists starter themes; applying is undoable and non-destructive (explicit overrides survive)
• Generators become theme-aware (SPEC §14.8) and emit generation_meta so regeneration preserves overrides
• ComponentInstance implemented for HeroTitleButton and ChapterThumbnailTile