Decision procedure for choosing which machine payment protocol to integrate: x402, FADP, MPP/Tempo, or plain API-key bil

Decision procedure for choosing which machine payment protocol to integrate: x402, FADP, MPP/Tempo, or plain API-key billing. Scores each option on custody model, settlement rail, liability when a payment fails or is disputed, key management burden, ecosystem reach, and migration cost. Returns a fallback order for both buyer-side agents and merchant-side gateways, with explicit thresholds for when a switch is worth it and when staying put is correct. Blunt and executable, not marketing.