RecordAttestation and record_hash are declared but unimplemented

[macanderson]

Noticed while implementing frame-layer attestation (PR #87).

The lifecycle profile declares:

• record_hash as the sha256:<hex> over the RFC 8785 (JCS) canonicalisation of a record with its own record_hash omitted from the preimage;
• RecordAttestation as a detached Ed25519 signature over that hash.

Both are types and prose. There is no JCS canonicalisation code, no record hashing, and no signature verification anywhere in the workspace — before PR #87 there was no cryptographic dependency at all. docs/profiles/context-exchange-provider.md LF1 promises tests/fixtures/ holds "golden JCS/record_hash vectors", and tests/fixtures/record-attestation.json exists as a shape example that nothing computes or checks.

So the record layer today has the same gap the frame layer just closed: a declared guarantee with no implementation behind it.

Needed: implement record_hash (JCS + the omit-self rule), implement RecordAttestation verification, and publish golden vectors so the LF1 claim becomes true. PR #87 adds sha2/ed25519-dalek behind the attestation feature, so the dependencies are already available — note JCS genuinely is the right choice here (an open-ended JSON document), unlike at the frame-provenance layer where ADR 0010 explains why it is not.

Definition of done

• record_hash is a callable library function in contextgraph-types, not a
  helper inside a test: RFC 8785 (JCS) canonicalization plus the omit-self
  rule, with the canonical preimage bytes exposed as well as the digest.
• The omit-self rule is decided in writing and implemented: whether
  record_hash is removed from the preimage or blanked, stated in an ADR
  under docs/adr/ with the reason, and matching what profile LH1 says.
• RFC 8785 conformance is demonstrated against the RFC's own published
  vectors — number serialization, UTF-16 key ordering, and string escaping —
  not only against values this repository generated.
• RecordAttestation verification exists (detached Ed25519), the exact
  signed preimage is pinned normatively in the profile, and verification
  recomputes the record's hash rather than trusting the stored member.
• tests/fixtures/ holds golden vectors that make the LF1 claim true: the
  canonical JCS text of every record fixture, and an attestation example
  carrying a real signature with the key published beside it.
• The conformance suite recomputes the fixtures through the library rather
  than through a second copy of the rule.
• The new code is compiled and run by CI (the features are off by default,
  so a job must enable them).
• A witness test fails without the change and passes with it, with both
  observations reported on the PR.
• Docs updated in the same PR: the profile, tests/fixtures/README.md, the
  crate README's feature list, and CHANGELOG.md under [Unreleased].
• Every finding noticed and not fixed is filed as its own issue with the
  triage label.