Deck check

Validate a deck against OPTCG construction rules — 50-card main deck, 1 Leader, every card shares a color with the Leader, max 4 copies per card ID, set/block-rotation legality. The validator returns every violation, not just the first.

Calls POST /api/v1/play/deck/validate — public, no-auth. Substrate-honest about the color check gracefully degrading while card_set_cards lacks a colors column; the response flags which checks were skipped and why.

Source-of-truth: docs/connections/the-play-substrate.md (S36 — the contract this page calls into) and docs/connections/the-play-structure.md (S37 — the structural follow-through this page is part of). The validator's pure-function form lives at apps/storefront/src/lib/play/deck-legality.ts. The contract endpoint is /api/v1/play/deck/validate.