/play/[deckSlug]/[variantSlug]/[slot]/ — single-slot Play-UI renderer with keyboard nav

Static route emitted for every slot in `SLOTS[variantSlug]`. Resolves the slot's `slug` from the registry, then dynamic-imports the per-slide component at `/src/components/slides/{variant}/{slot}-{slug}.astro` via `import.meta.glob` and renders it inside SlideCanvas; falls back to DecomposeFirstPlaceholder if the file doesn't exist. Today the route directly composes DididecksNav + DeckChrome + SlideCanvas inline; the target shape composes them via `DeckOverlay--Play-UI`, which also wraps the slide in `<section data-slot data-variant>` so SlideRankPill mounts work (closes A++.2 structurally).