Function clothoid_eval 

pub fn clothoid_eval(
    theta0: f64,
    kappa0: f64,
    kappa1: f64,
    s: f64,
) -> (f64, f64, f64)

Evaluate a clothoid curve at arc length s, starting from the origin with heading theta0, initial curvature kappa0, and sharpness kappa1.

Returns (x, y, theta) at arc length s.

Integration is performed via Simpson’s rule for numerical stability.