Stop the whole shipping order and release its physical reservation once.

• One step: check the shipment state and stop dispatch in one atomic step. Return 200 only when the order can no longer ship.
• Already shipped: when any line shipped, return 409 with error.code order_already_shipped and leave the order unchanged.
• Idempotency: external_ref. A replay returns the same receipt. A changed request under the same reference is 409 idempotency_conflict.
• Money and messages: do not refund and do not notify the customer.
• Retries: SKUU retries on network failure, 429 and any 5xx, up to 3 attempts with identical signed bytes. An incomplete cancellation can resume on a later event delivery.