Function ac_amplitude_drift_db 

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

AC-amplitude drift across a signal: dB difference between the single-bin amplitude of the SECOND half vs the FIRST half of signal.

≈0 dB means the tone has SETTLED (steady state). A large magnitude means the window is still inside a transient (e.g. a coupling cap still charging), so any gain/shape number taken on it is unreliable. This is the programmatic guard that a measurement window is past the settling transient.