HRV - Medera

HRV metrics are computed by _compute_hrv_metrics(peaks, fps) from rPPG inter-beat intervals.

Metric	Field	Description
Time-domain	`hrv_sdnn`	Standard deviation of NN intervals (ms)
Time-domain	`hrv_rmssd`	Root mean square of successive differences (ms)
Frequency-domain	`hrv_lf_hf_ratio`	Low-frequency / high-frequency ratio
Nonlinear	`hrv_sd1`	Poincaré plot short-term variability (ms)
Nonlinear	`hrv_sd2`	Poincaré plot long-term variability (ms)

Minimum duration: 60 s (min_hrv_duration). Lower SDNN and elevated LF/HF correlate with sympathetic dominance.