Function thd_db 

pub fn thd_db(
    signal: &[f64],
    fundamental_hz: f64,
    sample_rate: f64,
    n_harmonics: usize,
) -> f64

Compute THD (Total Harmonic Distortion) in dB.

THD = 10 * log10(sum(harmonic_powers) / fundamental_power)

Uses Blackman window for spectral analysis.