Settle an x402 payment (facilitator)

Re-verifies, then P2Flux's relayer settles the payment through P2FluxX402Splitter and pays the gas. success: true carries the transaction hash. errorReason: \"settlement_pending\" carries a real hash of a transaction still being mined: repeat the identical call once (the x402 middleware does) and it is resolved, never broadcast twice. A payment is reported settled ONCE: any further settle call for it - concurrent, later, or after the payment was settled by someone else - is refused with errorReason: "invalid_transaction_state", so one payment can never buy two responses. Every settlement carries the P2Flux fee, max(1%, 0.003 USDC) and never more than half the payment, paid out of the amount in the same transaction. Refusals are 200 with success: false, an empty transaction and the network.