Check an unsigned payment against the order's deposit before sending

Body { orderId, payment }. payment is what the wallet will sign, UNSIGNED: the transaction JSON {to, data, value, chainId} (data.deposit.payment.tx), an ethereum: (EIP-681) or monero: URI, the calldata, or an unsigned raw transaction. Never a signed transaction: one is refused with 400 signed_transaction and never stored or broadcast. Swapper reads the live order and compares the payment with its deposit (token, chain, recipient, exact amount). Send only if data.paymentCheck.safeToSend is true (it also requires data.payable true); data.paymentCheck.mismatches lists what differs. Reads only: creates nothing and moves no funds. Offline alternative: /swapper-verify.js (the same checks with Node, no dependencies).