Function rl_lowpass_filter 

pub fn rl_lowpass_filter(
    signal: &[f64],
    l_henrys: f64,
    r_ohms: f64,
    sample_rate: f64,
) -> Vec<f64>

Process a signal through an ideal first-order RL lowpass.

Uses bilinear transform for exact discrete-time response. H(s) = R / (R + sL) = 1 / (1 + sL/R) Time constant τ = L/R (same form as RC lowpass with τ = RC)

This gives fc = R / (2piL)