Function si_spline_interpolate 

#[unsafe(no_mangle)]
pub extern "C" fn si_spline_interpolate(
    pts: *const f64,
    t: f64,
    n: usize,
) -> f64

Linear interpolation along a polyline defined by control points. pts is a flat array of 2*n doubles (x0,y0, x1,y1, …). t is in [0, 1], mapping to the full arc length. For simplicity, this does uniform-parameter linear interpolation between segments.