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

Single mount point that composes the persistent overlay affordances riding over a Scroll-UI deck: the classifier pill (today), variant-cycling nav + counter (Phase 2 via PageAsDeckWrapper), presenter notes (Phase 3), and engagement telemetry (Phase D). Each is a named slot so consumers can rearrange or omit. The overlay element itself is layout-neutral (`display: contents`) — its children own their own fixed/absolute positioning. The Scroll-UI variant of this paired family; see DeckOverlay--Play-UI for the slot-per-route sibling.