Function zero_angle 

pub fn zero_angle(
    inputs: &BallisticInputs,
    trajectory_func: impl Fn(&BallisticInputs, f64) -> Result<f64, String> + Copy,
) -> Result<AngleResult, String>

Calculate zero angle using Brent’s method and Rust trajectory integration.

inputs.target_distance is meters and inputs.shooting_angle is radians. trajectory_func must return the projectile’s fixed-frame vertical height at the horizontal downrange distance inputs.target_distance.