docs(release): add v1 reviewer contract note

[stacknil]

Summary

• add a draft v1.0 reviewer-contract release note
• state the required boundary exactly: "This is a reviewer-contract release, not a production SIEM."
• link the note from the reviewer route and release gates, with regression coverage
• record the current validation snapshot without creating a tag or GitHub release

Validation

• python scripts/regenerate_artifacts.py --check (23 committed artifacts matched; 6 visual snapshots smoke-checked)
• python -m pytest tests/test_evidence_pipeline_schemas.py (3 passed)
• python -m pytest (175 passed)
• privacy/security pattern scan: no matches