Payment protocols.

trait PaymentProtocol {
    async fn create_challenge(...) -> Result<Challenge>;
    async fn verify_credential(...) -> Result<bool>;
    async fn settle(...) -> Result<Receipt>;
    async fn create_credential(...) -> Result<Credential>;
}

MPP, x402, AP2, Visa TAP, Mastercard Agent Pay, Tempo, and native TNZO are all first-class. The PaymentGateway picks the right protocol for a given PaymentProtocolId.

IdentityPaymentBinder looks up the payer DID, enforces DelegationScope::enforce_operation and the SpendingPolicyResolver, then routes to the protocol.

AP2 carts add an extra ceiling: the cart mandate is validated against both the delegation scope and the spending policy via Ap2Validator::validate_with_delegation_and_policy.