Add "proof" object as fourth structural component of a Record

[Jenziner]

Problem

RWP currently secures a Record's integrity via snapshotHash, but has no
structural place for an issuer's cryptographic signature over that
snapshot. Without this, no artifact derived from a Record can be verified
independently of the issuing system — a prerequisite for the Issued Copy
concept (recordweb/rwc#7).

Proposal

Add proof as an optional, repeatable structural component of a finalized
Record snapshot, modeled closely on W3C Verifiable Credential Data Integrity
1.0, without adopting VC terminology into RWP's core vocabulary.

Proposed schema addition

{
  "proof": [
    {
      "type": "DataIntegrityProof",
      "cryptosuite": "eddsa-jcs-2022",
      "created": "2026-07-23T08:00:00Z",
      "verificationMethod": "did:rwp:parlament.ch/system/rwp-node#key-1",
      "proofPurpose": "assertionMethod",
      "proofValue": "z...base58-multibase-signature..."
    }
  ]
}

Constraints

• proof is only permitted on Records in the finalized state (consistent
  with the existing principle that only finalized content is immutable and
  linkable).
• proof signs the canonical JCS serialization (RFC 8785) of the snapshot,
  consistent with RWP's existing hashing approach.
• Multiple proofs are permitted (e.g. institutional signature plus
  notarization signature), each independently verifiable.
• proof is additive to the snapshot; it does not alter snapshotHash
  computation, which continues to be computed over content only.

Open questions for CG discussion

• Which cryptosuite(s) should be mandatory-to-implement for RWP v1.0
  conformance?
• Is verificationMethod required to resolve via the same did:rwp
  resolution mechanism as Record identity, or can it reference external
  DID methods (e.g. did:web, did:key)?