Function circle_angle_to_parameter 

pub fn circle_angle_to_parameter(spans: usize, sweep: f64, angle: f64) -> f64

Parameter of the standard tangent-intersection rational quadratic arc construction (make_arc / make_revolution): a sweep split uniformly into spans segments, each with middle weight cos(segment/2). Maps an angle in [0, sweep] to the curve parameter in [0, 1] exactly.