docs(ci): document coverage lane

[EffortlessSteven]

Summary

Adds step 4 of the diffguard Codecov rollout: docs/ci/coverage.md documenting the claim boundary and separate proof lanes.

CI economics

• Default PR LEM impact: None; documentation only.
• Workflow touched: None; documentation file.
• Branch protection impact: None; documentation does not affect checks.
• Failure mode caught: None; this is reference material.
• Cheaper signal considered: No; documentation is a durable contract.
• Rollback path: Delete docs/ci/coverage.md; no other file depends on it yet.

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).

Documentation structure

• What it answers: execution-surface coverage
• What it does not answer: 9 separate categories of non-coverage claims
• Separate proof lanes: test, conformance, golden, mutation, fuzz
• Workflow triggers: main pushes, manual, labeled PRs
• Configuration: target, threshold, comments, annotations
• Artifacts: coverage.json, coverage.txt, lcov.info, coverage-receipt.json
• Tool: cargo-llvm-cov with LLVM profiling

Validation

• documentation is clear and exhaustive
• claim boundary is explicit and specific
• separate proof lanes are documented
• 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: 63afd2aa-68eb-4600-b0a2-857d410e7fed

📥 Commits

Reviewing files that changed from the base of the PR and between 3e1d9e1 and 65da2a3.

📒 Files selected for processing (1)

• docs/ci/coverage.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

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

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