SlideRankPill — floating five-button classifier for the active slot (rename pending → SlideClassifierPill)

Bottom-right floating pill that shows the active slot's number + title and exposes five enum buttons — Urgent / Could-be-better / Passable / Perfect / Pending. In Scroll-UI it discovers the active slot via IntersectionObserver over `section[data-slot][data-variant]` tags; in Play-UI the DeckOverlay--Play-UI wraps the slide in such a section so exactly one is in view. Hydrates ranks from `GET /api/slide-rank` on mount, posts optimistically on click, fails-soft in production builds. Rename to `SlideClassifierPill` is queued — 'classify' matches the cognitive act more accurately than 'rank' (the enum is a finite tag set, not an ordering).