结账：风险、测试与操作
定义了一个服务器权威的结账工作流程，包含定价快照、幂等的提供者会话、经过验证的异步履行、库存或权益保障以及对账证据。
Marketplace flows separate order, payment, payout, refund, and dispute states with explicit handling for uncertain payment-provider outcomes so partial failures never double-charge or double-pay sellers.