DeckChrome — themable floating navigation capsule for Play-UI (and eventually Scroll-UI variant cycling)

Floating bottom-right capsule with paginator, ← / → slot nav, TOC link, optional ↑ ↓ variant cycling, and the full keyboard contract (← → Space PageUp/Down Home End F C T Esc). Replaces v0.1's heavy dark PlayChrome bar. Structure / behavior / styling are separated by discipline: structure is the Astro template, behavior is the inline keyboard script and `ddd:section-*` event dispatch, styling reads only `--ddd-chrome-*` CSS custom properties (neutral defaults inline; consumer overrides at any wrapping element). Lives inside `DeckOverlay--Play-UI` as the default `nav` slot today; will also serve as the variant-cycling element of `DeckOverlay--Scroll-UI` under Phase 2.