ci(codecov): add coverage receipt artifact

[EffortlessSteven]

Summary

Adds step 5 of the diffguard Codecov rollout: coverage receipt artifact and step summary for durable proof and artifact reporting.

CI economics

• Default PR LEM impact: None; adds a step to the coverage job.
• Workflow touched: Updates .github/workflows/coverage.yml to include receipt generation and step summary.
• Branch protection impact: None; adds reporting, not requirements.
• Failure mode caught: Detects when coverage artifacts are missing or incomplete.
• Cheaper signal considered: Receipt is a few KiB of JSON; cheaper than uploading raw reports externally.
• Rollback path: Remove the receipt generation and summary steps; Codecov still works normally.

Claim boundary

Codecov coverage is Rust execution-surface evidence only. It does not prove:

• diff parser correctness
• rule evaluation correctness
• inline suppression correctness
• renderer completeness (SARIF, JUnit, Checkstyle, GitLab, CSV, TSV, sensor)
• LSP diagnostic correctness
• false-positive baseline or trend correctness
• mutation adequacy
• fuzz robustness
• release readiness

Those are separate proof lanes (test, conformance, golden, mutation, fuzz).

Enhancements

1. Coverage receipt: target/coverage/coverage-receipt.json

   • Documents the claim boundary explicitly
   • Lists which artifacts were generated
   • Schema version for future evolution

2. Artifact updates:

   • Includes coverage-receipt.json in the uploaded artifacts
   • 14-day retention for analysis and troubleshooting

3. Step summary:

   • GitHub Actions native artifact reporting via $GITHUB_STEP_SUMMARY
   • Clear table of artifact status
   • Explicit claim boundary inline

Validation

• receipt generation is idempotent and JSON-valid
• receipt includes all required fields (schema_version, repo, lane, tool, flag, claim_boundary)
• step summary renders correctly in GitHub UI
• artifacts are uploaded with correct paths
• no trailing whitespace or formatting issues

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 1fc9058d-54b8-4849-b8b3-86d3511705d5

📥 Commits

Reviewing files that changed from the base of the PR and between 3e1d9e1 and a2401bf.

📒 Files selected for processing (1)

• .github/workflows/coverage.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/codecov-receipt

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