Harden Core public verification and claim boundaries

[Riverbraid]

What changed

• Removes four zero-byte, unconsumed contract/fixture placeholders.
• Adds fixtures/README.md with the rule that fixtures require a declared consumer, contract, expected behavior, and evidence relationship.
• Adds claim-ceiling.json as a machine-readable allowed/refused-claim declaration.
• Aligns package.json with Node 20.11.0 and npm 10.2.4, while adding bounded verify and verify:compatibility commands.
• Reconciles Ring 0 repository authority with the dependent Ring 2 compatibility gate.
• Retires Proposed Phase 15 from current authority vocabulary.
• Routes public evaluation explicitly to Riverbraid-Evaluation-Kit.
• Reclassifies the existing mutable-sibling workflow as a live compatibility check rather than pinned reproduction.
• Pins the workflow to ubuntu-24.04, actions/checkout@11d5960a326750d5838078e36cf38b85af677262, and actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020.

Exact-head validation

Current head:

699567de01e83494a8f4a00b503b322ff61b410c

GitHub Actions run 30285743307 completed successfully.

The verify-core-live-compatibility job completed:

• exact action checkout;
• Node 20.11.0 setup;
• current Ring 0 sibling default-branch cloning;
• live compatibility audit;
• bounded final-status emission.

This supports only the exact claim that the PR head's live mutable-default-branch compatibility workflow executed successfully in the observed environment. It does not convert that workflow into the pinned Evaluation Kit profile.

Boundaries

This PR does not claim certification, production readiness, external audit, complete AI safety, absolute security, registry freshness, adoption, full-constellation verification, independent reproduction, or absence of defects.

It does not mutate Evaluation Kit registry pins, releases, tags, repository settings, or the F0–F4 control package.

The documentation and workflow-classification portion of issue #6 is addressed. Any future pinned Core compatibility profile remains a separate evidence and authority decision.