Function fgrad_linext_inplace_f32 

pub(crate) fn fgrad_linext_inplace_f32(
    gx: &mut [f32],
    gy: &mut [f32],
    gz: &mut [f32],
    x: &[f32],
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f32,
    vsy: f32,
    vsz: f32,
)

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