Buy a member's offering over x402

An x402 v2 resource (coinbase/x402 specs/transports-v2/http.md). Without a payment header it answers 402 with PaymentRequirements (in the PAYMENT-REQUIRED header and the body) whose payTo is the SELLING MEMBER's own address: Base USDC (exact, EIP-3009) and, when the member takes it, Solana USDC (exact, fee payer from the CDP facilitator). Retry with PAYMENT-SIGNATURE (or the v1 X-PAYMENT) header: IC checks the payment itself, settles through the CDP facilitator straight into the member's wallet, fulfils (file link, booking, paid inbox message, the member's webhook, or manual) and returns the receipt with PAYMENT-RESPONSE / X-PAYMENT-RESPONSE. The same payment replayed returns the same receipt and is never fulfilled twice. A fulfilment that fails after settlement still returns the receipt (status fulfilment_failed); the member is told. 202 = settlement_pending: retry the same request, never pay again. IC holds no funds and takes no fee. Unknown handle, unknown slug, paused and taken-down offerings all answer the same 404. MCP discovery: ic_offerings_get. 404 unless offerings are enabled.