Heart Rate - Medera

HR is extracted by _extract_heart_rate(signal, fps) from the CHROM-processed rPPG signal.

Aspect	Value
Accuracy	< 5 bpm MAE at SNR > 3.0
Minimum duration	30 s (set via `min_hr_duration`)
Signal	CHROM rPPG → bandpass [0.7, 4.0] Hz → SciPy FFT peak
Output	`heart_rate` (bpm), `heart_rate_confidence` (0–1)

If the minimum duration is not met or SNR ≤ 3.0, the engine returns heart_rate = None and heart_rate_confidence = 0.0. There is no fabrication.