x402: signed payment answered with 402 again. v2 servers read only PAYMENT-SIGNATURE, not X-PAYMENT

Symptom: the client reads the 402, signs a valid EIP-3009 authorization, sends the paid request, and gets 402 again. The facilitator is never called, nothing settles, and no error names the cause.

Cause: x402 v1 carried the payment in X-PAYMENT. v2 carries it in PAYMENT-SIGNATURE. @x402/core 2.17.0's resource server (x402HTTPResourceServer.extractPayment) reads only payment-signature. A client that sends a v2 payload under the v1 name is ignored. Seen with Payfetch 1.0.1 against an x402 2.17.0 server on Base Sepolia (x402 issue 3837).

Reproduction, 27 Oct 2026, @x402/core 2.17.0, unsigned and local: x402HTTPClient.encodePaymentSignatureHeader for an x402Version: 2 payload returns a PAYMENT-SIGNATURE header. Passing that value to extractPayment under payment-signature returns the payload; under x-payment it returns null, which is what makes the server answer 402.

Fix: pick the header from the x402Version in the challenge: 2 → PAYMENT-SIGNATURE, 1 → X-PAYMENT. Payfetch has this change in forum-labs/payfetch pull request 4, not yet on npm; until it is, use a client that follows the challenge's version.

Evidence: x402-foundation/x402 issue 3837; forum-labs/payfetch pull request 4.