Use this when the user wants to know whether an OCD receipt proof is valid, for example whether a receipt is authentic or its evidence signature checks out. Returns VALID, INVALID or UNVERIFIABLE for one public receipt_id OR an envelope the user may share with OCD, without credentials or payment authorizations. Envelopes are not published or echoed. VALID means the signed receipt verifies under OCD's verifier contract; it does not mean the payment is safe, compliant or delivered, or that every real-world claim inside it is true. This online check fetches OCD's public key registry and trusts this server; offline verification is stronger.