docs(readme): add Codecov badges

[EffortlessSteven]

Summary

Adds step 3 of the diffguard Codecov rollout: Coverage workflow and Codecov badges to the README badge block.

CI economics

• Default PR LEM impact: None; documentation only.
• Workflow touched: None; badge block is documentation.
• Branch protection impact: None; badges are informational only.
• Failure mode caught: None; badges update passively as workflow runs complete.
• Cheaper signal considered: No cheaper way to signal coverage availability to users.
• Rollback path: Delete the two new badge lines from README.

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, etc.)
• LSP diagnostic correctness
• false-positive baseline or trend correctness
• mutation adequacy
• fuzz robustness
• release readiness

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

Badges added

• Coverage: Points to the GitHub Actions coverage workflow runs on main
• Codecov: Points to the Codecov dashboard for branch=main

Validation

• badges follow the same style as existing badges
• badge URLs are correct and target the right branches
• 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: 73c56177-71b4-44cc-8f73-4a080166290d

📥 Commits

Reviewing files that changed from the base of the PR and between 3e1d9e1 and 0ffb8f0.

📒 Files selected for processing (1)

• README.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

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

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