Create a storefront order

Finalize a storefront cart into a Foundry order. The storefront owns payment processing — Foundry just records the order, server-recomputes totals from current ChannelSku.price values, and reserves inventory.

Returns { orderId, orderNumber, status, lookupToken }. The token is the customer's escape-hatch reference: storefronts give it to the customer (in the confirmation email, account page, wherever) so they can fetch order status via Phase 3's GET /storefront/orders/:lookupToken without a Foundry account.

Pass Idempotency-Key to make retries safe — replays with the same key + body return the cached 2xx; same key + different body returns 409 to surface the misuse.