docs(contract): document v0.6 to v1 artifact diff

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Summary

• add the concrete v0.6.0 to v1 artifact contract diff for the fourth and fifth demos
• document unchanged fourth-demo paths, additive fifth-demo artifacts, schema coverage, semantic differences, and consumer guidance
• link the diff from the reviewer route, freeze/readiness docs, and protect it with reviewer-doc tests

Validation

• python -m pytest tests/test_reviewer_docs.py tests/test_markdown_links.py (17 passed)
• python scripts/regenerate_artifacts.py --check (23 stable artifacts matched; 6 visual snapshots smoke-checked)
• python -m pytest (174 passed)
• privacy/security pattern scan: no matches