Function single_bin_amplitude 

pub fn single_bin_amplitude(
    signal: &[f64],
    freq_hz: f64,
    sample_rate: f64,
) -> f64

Single-bin DFT amplitude (peak, in signal units) of the component at exactly freq_hz.

Goertzel-style projection onto cos/sin at freq_hz. The DC offset is removed first, so the result is the AC amplitude AT THE TEST FREQUENCY and is immune to any DC pedestal (a device sitting at its bias point) or slow drift baseline. This is the steady-state level measurement that normalized_rms_error_db cannot provide.