docs(plan-2e): draft q2-slides + revealjs sibling-format migration

Migration plan for moving ReactAstSlideRenderer.tsx (SlideAst carousel)
and RevealjsReactAstSlideRenderer.tsx (RevealjsSlideAst) into a shared
hub-client/src/components/render/q2-slides/ directory, completing the
hub-client format-restructure that 2pre started.

§2 (two registries sharing leaves) is locked: q2SlidesRegistry and
revealjsRegistry both spread the same blocks/inlines modules and differ
only on the 'Ast' document-root entry. §1 (iframe boundary), §3
(SlideContext shape), §4 (asset resolution), §5 (editing wiring), and
§6 (migration strategy) are open with recommendations.

Phased shim approach (Phases 9-13) mirrors 2pre's pattern: build
q2-slides/ behind a re-export shim, migrate four consumers
(ReactRenderer, RevealjsReactAstSlideRenderer, useCursorToSlide,
useSlideThumbnails) one commit at a time, then delete the shim. Depends
on Plan 2D Phase 6.0 (framework/meta.ts) landing first.

Author: gordonwoodhull