Pay a receipt, trigger the effect.

Every flow is pay → receipt → side effect. The frontend or an agent triggers an MPP charge on Tempo Moderato; the server verifies the on-chain receipt; only then does the side effect run — here, unlocking a downloadable prompt pack. HTTP 402 and WWW-Authenticate are preserved end-to-end so mppx can retry-with-payment.

Live mode requires an MPP_RECIPIENT secret plus mppx server wiring. This demo runs the full pattern in simulate so the flow is auditable without spending testnet funds.