Checkout is the point where agent-ready commerce stops being mostly read-only. Discovery can be a projection. Comparison can be a query. Policy quotation can be a controlled answer backed by source evidence. Checkout is different. Checkout mutates commercial state. A cart changes. A price may be selected or locked. Inventory may be revalidated or reserved. Shipping options may be calculated. Policy coverage may become a precondition. A checkout snapshot may be created. Payment authority ma...