Function compute_spin_drift_with_decay 

pub fn compute_spin_drift_with_decay(
    time_s: f64,
    stability: f64,
    twist_rate: f64,
    is_twist_right: bool,
    decay_factor: Option<f64>,
) -> f64

Calculate Litz spin drift with an optional extra retained-spin multiplier.

Canonical solver paths pass no multiplier because the empirical Litz time exponent already reflects real spin history. Some remains for source compatibility and experimental callers intentionally applying an extra scalar; canonical solvers must not use it for spin decay.