Buy (or check) an x402 session

Without a session or payment this answers 402 with the x402 body: accepts[0] gives the exact amount, payTo (QPayhub) and extra.{sellerId, resourceId, settlement:'contract'}, and paymentTicket carries a nonce. Pay on-chain with QPAYHUB.Pay (inputType 1; seller = sellerId, resource = sha256(resourceId), the ticket's nonce, exactly that amount), then repeat the request with X-PAYMENT: base64(JSON { x402Version: 2, resource, accepted, payload: { txHash, ticket }, extensions }). The server finds the payment in QPayhub's receipt, checks payer, seller, resource, nonce and amount, counts each payment once, and returns 200 with X-ACCESS-GRANT and X-ACCESS-GRANT-EXPIRES. Send the grant as X-ACCESS-GRANT on later requests. If the transaction is not confirmed yet the 402 says invalid_transaction_state: wait a few seconds and send the same header again. Inside a session there is no per-call charge for split quotes or arbitrage results, and the rate limit is the keyed one (ten times the free limit), counted for the session rather than the IP address.