Function fgrad_linext_f32 

pub(crate) fn fgrad_linext_f32(
    x: &[f32],
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f32,
    vsy: f32,
    vsz: f32,
) -> (Vec<f32>, Vec<f32>, Vec<f32>)

Forward difference gradient with linear extrapolation boundary conditions (f32) Matches MATLAB’s gradf behavior: dx(end) = dx(end-1)