Add to the balance with USDC on Base (x402 v2)

x402 v2 over HTTP. Without a PAYMENT-SIGNATURE header the answer is 402 with the payment terms in the PAYMENT-REQUIRED header (base64 JSON; also in the body as payment_required): exact scheme, USDC on Base (eip155:8453), paid to Phoenix Labs. Sign the EIP-3009 transfer and repeat the request with the base64 PaymentPayload in PAYMENT-SIGNATURE. A facilitator settles it (no gas needed from the payer), the transfer is confirmed on-chain, and the balance is credited 1 USDC = $1. The receipt comes back in PAYMENT-RESPONSE. $1 to $200. Refunds of unused USDC balance go back to the paying address in USDC.