Add reusable workflow and contract for mirror implementation conformance

[Copilot]

Mirror Conformance Implementation

This PR implements infrastructure-enforced mirror conformance for Base120 language implementations, making mirror compliance mandatory rather than advisory.

Changes

1. Conformance Contract (mirrors/CONFORMANCE_CONTRACT.md)

   • Defines byte-for-byte golden corpus conformance requirements
   • Specifies mirror lifecycle states: draft → review → approved → deprecated → removed
   • Documents corpus diff validation criteria with clear pass/fail conditions
   • Includes troubleshooting guidance and examples
   • Requires BASE120_FIXED_TIMESTAMP support for deterministic testing

2. Reusable CI Workflow (.github/workflows/mirror-conformance.yml)

   • Reusable GitHub Actions workflow for automatic validation
   • Clones canonical corpus and validates mirror implementations
   • Compares outputs byte-for-byte and verifies determinism (3 iterations)
   • Posts detailed PR comments with diagnostics
   • Blocks merge on conformance failures
   • Robust error handling for edge cases

3. Mirror Registry (mirrors/README.md)

   • Step-by-step guide for creating mirror implementations
   • CI integration examples for various languages
   • Certification process documentation
   • Troubleshooting for common issues

4. Documentation Updates (.github/workflows/README.md)

   • Added mirror-conformance workflow documentation
   • Updated changelog and related references

Rebase on Main

This branch has been rebased on top of main to incorporate the Golden Corpus Determinism test fix from PR #34 (commit 3fbe537). The fix updates the governance-invariants workflow to use pytest-json-report for deterministic test output comparison.

Testing

✅ All 42 tests pass
✅ Corpus tests validated
✅ YAML syntax confirmed
✅ Follows governance patterns from PR #26

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Harden Mirror Conformance via Explicit Contract and CI Enforceability</issue_title>
<issue_description>Redesign mirror validation as a strict infrastructure contract:

• Author mirrors/CONFORMANCE_CONTRACT.md describing exact golden corpus conformance checks and lifecycle (draft → approved → deprecated)
• All mirror repos must pass corpus diff validation before approval/certification
• Provide a reusable GitHub Actions workflow that mirror repos can import to automate (or, if not possible, to facilitate manual) compatibility tests against the canonical corpus
• Wire CI to reject/alert PRs that break conformance

Outcome: Mirror compliance is infra-enforced, not advisory or optional.</issue_description>

<agent_instructions>Implement this in phases:

Phase 1: Create mirrors/CONFORMANCE_CONTRACT.md

• Document golden corpus conformance requirements
• Define mirror lifecycle states (draft → approved → deprecated)
• Specify corpus diff validation criteria
• Include examples and troubleshooting

Phase 2: Create reusable GitHub Actions workflow

• Author .github/workflows/mirror-conformance.yml as a reusable workflow
• Implement corpus diff validation against canonical base120 corpus
• Add automatic PR comments with diff results
• Wire to CI to reject PRs that break conformance

Phase 3: Integration

• Update mirrors/README.md to reference CONFORMANCE_CONTRACT.md
• Add workflow usage example for mirror repos
• Document certification process

Follow existing governance patterns from PR #26. Ensure determinism via BASE120_FIXED_TIMESTAMP.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Harden Mirror Conformance via Explicit Contract and CI Enforceability #20

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[github-actions]

🔍 Governance Classification

Change Class: editorial (Impact Level: 2)

Description: Documentation restructuring or content expansion

Evidence Requirements

• Documentation builds successfully (if applicable)

Review Requirements

• Minimum Reviewers: 0 external + CODEOWNER approval
• Audit Update Required: ❌ No

Next Steps

1. ✅ Ensure documentation is clear and accurate
2. ✅ Wait for required approvals (0 + CODEOWNER)
3. ✅ Address any CI check failures

---

📚 See GOVERNANCE.md for detailed requirements.

[hummbl-dev]

approved

[hummbl-dev]

@copilot The Golden Corpus Determinism test is failing on this PR because it's using the old workflow from before the fix was merged to main.

The fix was merged in PR #34, and main now has the corrected workflow file. Please rebase this PR on top of the latest main branch to pick up the fixed workflow, which will resolve the failing check.

[Copilot]

@copilot The Golden Corpus Determinism test is failing on this PR because it's using the old workflow from before the fix was merged to main.

The fix was merged in PR #34, and main now has the corr...

Done! This branch has been rebased on top of main and now includes the Golden Corpus Determinism test fix from PR #34 (commit 04cd5ae). The workflow now uses pytest-json-report for deterministic output comparison. All 42 tests pass.