Function calculate_dynamic_stability 

pub fn calculate_dynamic_stability(
    bullet_mass_grains: f64,
    velocity_mps: f64,
    spin_rate_rad_s: f64,
    caliber_inches: f64,
    length_inches: f64,
    air_density_kg_m3: f64,
) -> f64

Calculate current gyroscopic stability from retained spin using the Miller formula.

The effective twist is back-calculated from the supplied roll rate and current airspeed, then corrected for current velocity and density. Callers that do not integrate roll should pass the muzzle-set spin rate while allowing velocity_mps and air_density_kg_m3 to evolve.