feat: add Codecov coverage badge to README

[GeorgePearse]

Adds Codecov coverage badge to README.md to display test coverage metrics and link to the Codecov dashboard.

Summary

This PR adds a Codecov badge alongside the existing project status badges in the README, providing readers with visibility into the test coverage of the repository.

Changes

• ✨ Added Codecov badge to README.md (line 13)
• Badge displays coverage percentage and links to full coverage report

Coverage Badge Details

• Format: Codecov standard badge with master branch coverage
• Links to: https://codecov.io/gh/GeorgePearse/bayesian_filters
• Updates: Automatically after each CI run on master branch
• Integration: Leverages existing Codecov integration in test.yml and test-coverage.yml workflows

Testing

✅ All 231 tests pass
✅ Pre-commit hooks pass
✅ Markdown renders correctly

Implementation Notes

• No changes to CI/CD workflows (already configured)
• No additional dependencies required
• Codecov token not required for public repo (badge updates work without CODECOV_TOKEN secret)
• Badge will display "unknown" until first successful coverage upload

Future Enhancements

• Could add codecov.yml configuration file for fine-grained control
• Could add separate badges for different branches
• Could customize precision (0-2 decimal places) if needed

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.