feat(reader): enhance annotation type mapping and waveform detection
- Detect P, QRS, T, and U wave regions using WFON markers
- Distinguish waveform peaks (P_PEAK, R_PEAK, T_PEAK, U_PEAK)
- Support multiple QRS beat annotation types
- Add mappings for standalone rhythm and event annotations
- Return empty string for invalid or unmatched annotations
- Improve NULL and boundary checks