Freeze reviewer artifact contract

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Summary

• Expands the reviewer pack artifact naming contract into an explicit stable artifact table
• Marks the reviewer pack as the artifact naming contract from README
• Adds reviewer-doc regression coverage for every stable reviewer-visible artifact path

Validation

• pytest tests/test_reviewer_docs.py --basetemp .pytest-artifacts-artifact-contract
• pytest --basetemp .pytest-artifacts-full-20260526-1
• git diff --cached --check