/play/[deckSlug]/[variantSlug]/ — variant-index redirect to slot 01

Enumerates every (deck, variant) pair from the consumer's deck registry and 302s to /play/{deck}/{variant}/01/. Today the redirect is unconditional; A++.1 will gate it on `SLOTS[variantSlug]?.length` and render a friendly 'no slots yet' panel for variants without entries (currently chroma's proto, enhanced-v1, enhanced-v2 — all return 404s after the redirect because `[slot]` only emits paths from SLOTS).