Function calculate_spin_damping_moment 

pub fn calculate_spin_damping_moment(
    spin_rate_rad_s: f64,
    velocity_mps: f64,
    air_density_kg_m3: f64,
    caliber_m: f64,
    length_m: f64,
    decay_params: &SpinDecayParameters,
) -> f64

Calculate the magnitude of the aerodynamic moment opposing spin.

Uses the conventional roll-damping relation M = 1/4 * rho * V * S * d^2 * |C_lp| * |p|, where S is projectile reference area. Here C_lp is the derivative with respect to reduced spin p*d/(2*V); conventions using p*d/V report a coefficient smaller by a factor of two. The returned value is nonnegative; calculate_spin_decay_rate applies the direction that opposes the signed spin rate.