feat(ecg): expand supported heart rate detection range
- Reduce minimum RR interval from 300 ms to 200 ms
- Increase maximum RR interval from 2.0 s to 4.0 s
- Expand supported heart rate range from 30–200 BPM to 15–300 BPM
- Update maximum RR interval validation to allow up to 5.0 s
- Retain the 200 ms QRS refractory period