Function calculate_spin_decay_rate 

pub fn calculate_spin_decay_rate(
    spin_rate_rad_s: f64,
    velocity_mps: f64,
    air_density_kg_m3: f64,
    mass_grains: f64,
    caliber_inches: f64,
    length_inches: f64,
    decay_params: &SpinDecayParameters,
    bullet_shape: &str,
) -> f64

Calculate the rate of spin decay in rad/s²