Add Code Coverage Reporting to CI Pipeline

[Samuel1505]

Summary

This PR extends the GitHub Actions CI pipeline to measure and report code coverage using cargo-llvm-cov, ensuring code quality standards are maintained.

Changes

• Added new coverage job to CI workflow that runs on ubuntu-latest
• Installs and runs cargo-llvm-cov to generate coverage reports in LCOV format
• Implements coverage threshold check: CI fails if coverage drops below 60%
• Uploads coverage reports to Codecov for tracking and visualization
• Posts coverage summary comments on pull requests
• Adds Codecov badge to README for visibility

Technical Details

The coverage job:

1. Installs cargo-llvm-cov using the official install action
2. Generates coverage report with --lcov --output-path lcov.info
3. Extracts coverage percentage and validates against 60% threshold
4. Uploads to Codecov using the codecov-action
5. Posts a sticky PR comment with coverage summary and links

Benefits

• Automated coverage tracking on every push and PR
• Enforces minimum code quality standards (60% threshold)
• Provides visibility into coverage trends via Codecov dashboard
• Helps maintain and improve test coverage over time

closes #286