Require a re-mint when diagnostic_version increases

[coderabbitai]

Summary

Prevent a version-only manifest.diagnostic_version update from allowing a stale regression baseline to pass the CI coupling gate.

Rationale

The current gate can accept this sequence:

1. Diagnostic.version increases.
2. A pull request updates only manifest.diagnostic_version to the new value.
3. test_case_version, committed, and native remain unchanged.
4. The gate returns SKIP.

This allows committed and native baseline data produced by the earlier diagnostic version to remain in use.

This behaviour predates #854. It is intentionally out of scope for that pull request.

Required changes

Update the regression gate policy so that an increase in diagnostic_version requires a re-mint-authorised baseline update. A test_case_version increase is one possible authorisation mechanism.

Add regression coverage for a version-only manifest edit. The test must verify that the gate does not return SKIP when diagnostic_version increases without the required baseline update.

Affected areas

• packages/climate-ref-core/src/climate_ref_core/regression/gate.py
• packages/climate-ref-core/tests/unit/regression/test_gate.py
• Related regression-gate documentation, if the policy changes documented behaviour.

Acceptance criteria

• A diagnostic_version increase cannot pass with unchanged test_case_version, committed, and native baseline data.
• The gate emits a clear failure or required-action result for this case.
• Unit tests cover the version-only manifest update.
• Documentation describes the final policy.

Backlinks

• Pull request: Remove dead test-cases surface and simplify the CI coupling gate #854
• Review discussion: Remove dead test-cases surface and simplify the CI coupling gate #854 (comment)

Requested by @lewisjared.