feat(ecg): add RR statistics and rhythm completion detection
- Add RR interval and heart rate calculation from QRS annotations
- Store computed mean RR interval and BPM in EcgFeatures
- Integrate rhythm completion flow after R-peak detection
- Add T-wave end detection based on R-peak reference and HR window
- Prepare U-wave detection interface for future implementation
- Update beat MHR assignment to use computed BPM
- Enable format-on-save in VS Code settings
- Apply clang-format style updates across ECG processing and tests
- Reduce dataset validation sample limit for faster testing