Phase 1: Add code coverage reporting

[RAprogramm]

Task

Integrate cargo-llvm-cov for accurate code coverage and upload reports to Codecov.

Implementation

1. Add coverage job to reusable-ci.yml
2. Install cargo-llvm-cov
3. Generate coverage with --lcov output
4. Upload to Codecov
5. Add coverage badge to README

Benefits

• Visualize test coverage in PRs
• Track coverage trends
• Enforce quality standards

Acceptance Criteria

• Coverage job runs in CI
• Reports uploaded to Codecov
• Coverage badge in README
• Coverage visible in PR comments

Parent: #175