Function fft3d_real 

pub fn fft3d_real(
    data: &[f64],
    nx: usize,
    ny: usize,
    nz: usize,
) -> Vec<Complex64> 

3D FFT of real data (real-to-complex)

Returns complex array. Output shape is (nx, ny, nz) for simplicity (not the half-spectrum like numpy’s rfft).