feat(verify): serve the nightly ground-truth attestation as a first-class trust surface

[JSONbored]

Parent: #11066. Producer: the verify container class in metagraphed-infra.

Why this is a product surface and not an internal check

A registry's product is trust, and today we ask for it. verify_integration exists as a tool; what
it can return is bounded by having nothing independent to compare against.

An attestation that says "these values were re-derived last night by a second, independent
implementation, N sampled, N matched, here are the differences" is a claim that is (a) checkable by
the reader, (b) expensive for anyone else to copy, and (c) exactly the discipline that would have
caught Neon's bulk seed silently losing 459 history rows over 24 days — noticed after nine.

Scope

• A dated attestation artifact: what was sampled, the method, what matched, what did not, with the
  actual differing values
• Served as its own route and surfaced through verify_integration
• Linked from the surfaces whose numbers it covers, so the attestation is reachable from the claim
  it supports

The honesty requirement

Publish the diffs, including the ones that make us look bad. An attestation that only ever reports
success is not evidence and readers are right to discount it. If the nightly run finds a
disagreement, the artifact says so and the surface reflects it.

Acceptance

• Attestation served with method, sample and results, dated
• A non-empty diff is visible to a reader, not just to us
• verify_integration returns the attestation rather than a self-consistency check
• The surface degrades to a schema-stable empty when no attestation exists yet

[linear-code]

JSO-8212