Add predicate: EIP-712 Payload Attestation (secp256k1 typed-data receipt over exact bytes)

[0rkz]

What: A predicate type carrying an EIP-712 PayloadAttestation — a secp256k1 typed-data signature over the exact bytes of a served payload with a signer-declared validity deadline — inside a Statement v1, so receipts produced in the Ethereum/x402 ecosystem can be consumed by in-toto/DSSE policy tooling without re-signing. Type URI is under our own domain (https://payperbyte.io/attestation/eip712-payload-attestation/v1), so this PR only asks for directory listing/vetting, not a namespace grant.

Why existing predicates don't fit: none carries an independently verifiable third-party typed-data signature whose key is recovered from the signature itself; SLSA Provenance/SCAI/Simple Verification Result describe builds or evaluations, not "which key signed exactly these bytes and until when". The predicate deliberately nests the EIP-712 signature as content (DSSE signs the Statement; the nested signature verifies on its own terms).

Policy questions it answers: is publisher an accepted key for this resource; does keccak256(bytes) equal the signed payloadHash; is the receipt within its deadline; under which EIP-712 domain was it signed.

Scope: authenticity + tamper-evidence of exact bytes — explicitly not correctness/truth of the payload (stated normatively in the spec).

Example: complete Statement with a real, re-verifiable signature (ephemeral example key); verification sketch included.

Disclosure: authored by BYTEDev Inc (PayPerByte/ForeSeal), whose HTTP receipt uses this struct in production; the predicate is generic and not tied to our service.