DeckOverlay--Play-UI — the floating overlay layer for Play-mode decks

Single mount point for /play/[deck]/[variant]/[slot]/ that composes DeckChrome (paginator + slot-step nav + keyboard contract) and SlideRankPill (classifier) above the active slide, plus named slots for Phase 3 presenter notes and Phase D telemetry. Wraps the slide in a `<section data-slot data-variant>` so SlideRankPill's existing IntersectionObserver finds exactly one section — the play-mode adapter is the section-wrap itself, no special play-mode prop required. Sets `data-play-root` + `data-chrome-hidden` so DeckChrome's `C` keypress fades the whole overlay together.