Doordash Group Checkout Gate

Live allergen re-check for DoorDash CLI group orders. When a checkout runs for a cart that has a doordash-group-orders round ledger (.dd/round-*.json), this hook itself runs 'dd-cli cart show', joins cart lines to eaters via the person-to-cart-item-id ledger, and re-derives allergen conflicts against each eater's entry in the team-food.json roster - blocking with exit 2 on a live anaphylaxis-tier match. Verifies reality at checkout time instead of trusting a flag file: a skipped vetting step or a cart edited after review cannot pass silently. Only activates for carts with a round ledger; solo orders are doordash-allergy-shield territory.