Integration with x402 Headers

• Offer: Add X-402-AAR-OFFER (base64url JSON) alongside the standard x402 pricing header.

• Payment Retry: X-402-PAYMENT carries the normal receipt envelope; include the AAR receipt binding under receipt.

• Errors: X-402-AAR-ERROR when AAR‑specific validation fails.

Example (illustrative):

HTTP/1.1 402 Payment Required
X-402-OFFER: <base64url(x402-offer-json)>
X-402-AAR-OFFER: <base64url(aar-offer-json)>
Content-Type: application/json

Client retry with payment:

GET /resource
X-402-PAYMENT: <base64url({
  "receipt": { ... AAR binding ... },
  "signature": "0x..."
})>