fix: reduce behavioral relation validation complexity

[Brad-Edwards]

Summary

Refactors behavioral-relation catalog reference validation into focused helpers without changing validation behavior, reducing the SonarCloud-reported complexity below the configured threshold.

Related issues

• Addresses the SonarCloud quality-gate finding reported on Dev #977.

Changes

• Extract bibliography, relation, claim-surface, and worked-example reference checks from the catalog model validator.

Test plan

• Focused behavioral-relations tests pass
• Repository lint passes
• Repository policy check passes
• Full verification could not complete locally because the Ground Control governance endpoint did not respond.

Checklist

• Code follows project coding standards
• No semantic behavior change
• No published contract schema changes
• PR title is a Conventional Commit
• No architectural documentation update needed

Notes for review

The full pre-commit and verification gates reached the repository governance client, which blocked on the unavailable local Ground Control endpoint. The targeted behavior suite and lint checks passed.