Menu role model: semantic roles over physical domains

[ScottMorris]

Part of the rich menu editor epic. Plan: docs/rich-menu-editor-plan.md §2 decision 3. Lands with whichever of Slice A / Slice C ships first.

Menus carry semantic intent; physical placement becomes a backend mapping.

• MenuRole enum (root, title-select, chapter, setup, extras, popup) on MenuDocument
• Add generatorKind (or per-generator ids) to MenuGenerationMeta — today every generator writes generatorId: 'menu-workspace', which cannot discriminate chapter vs setup menus
• Role inference for existing projects on load: enriched metadata → interaction-content detection (PlayChapter → chapter, SetAudioStream/SetSubtitleStream → setup) → entry-menu-only Root → TitleSelect fallback; user-reassignable afterwards
• DVD backend maps role → MenuDomain (VMGM/VTSM placement)
• Menu map and generators group by role; terminology map renders role names per format
• popup authorable only when the active FormatProfile supports it (none until BD)

Reconciles with docs/blu-ray-integration-plan.md §1.7: BdMenuType becomes the BD backend's mapping of role, as MenuDomain is DVD's.