Integrate cargo-llvm-cov into CI for delta coverage reporting

[iamclaude697]

Context

CONTRIBUTING.md requires 95% delta coverage on PRs, but no coverage tooling is currently integrated into CI. Test counts and qualitative evidence are provided in PR descriptions, but quantitative measurement is not available.

Proposed implementation

1. Add cargo-llvm-cov to the CI pipeline
2. Configure delta coverage reporting (compare PR branch vs base branch)
3. Output coverage summary in CI logs and as a PR comment
4. Enforce 95% delta / 85% overall thresholds as CI gates

References

• PR feat: external framework modules — gRPC plugin host, Go/Python framework detection #16 review finding M6: quantitative coverage gate requires tooling
• CONTRIBUTING.md coverage gates: 95% delta, 85% overall, 100% on validation paths