Function update_spin_rate 

pub fn update_spin_rate(
    initial_spin_rad_s: f64,
    time_elapsed_s: f64,
    velocity_mps: f64,
    _air_density_kg_m3: f64,
    mass_grains: f64,
    _caliber_inches: f64,
    _length_inches: f64,
    decay_params: Option<&SpinDecayParameters>,
) -> f64

Calculate the spin rate after accounting for decay

Uses an empirical model based on published ballistics data. Real bullets typically lose 5-15% of spin over a 3-second flight.