Add CI Code-Coverage and Doc-Tests

[adriendellagaspera]

Background
While we have unit tests in place, we don’t yet track what percentage of code and examples are exercised. Code-coverage and doc-tests will help ensure long-term maintainability and correctness of our documentation.

Goals

• Measure and report Rust code coverage on each CI run.
• Run all examples in documentation as cargo test --doc.

Tasks

1. Integrate a coverage tool (e.g. grcov or Codecov + llvm-cov) into GitHub Actions.
2. Configure CI to upload and display coverage badges.
3. Ensure cargo test --doc passes, correcting any broken code snippets.
4. Write tests to verify coverage thresholds (e.g., > 90%).
5. Update README.md with a coverage badge and instructions on running doc-tests locally.