Function fgrad_masked 

fn fgrad_masked(
    f: &[f64],
    mask: &[u8],
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f64,
    vsy: f64,
    vsz: f64,
) -> (Vec<f64>, Vec<f64>, Vec<f64>)

Mask-adaptive forward gradient (matching MATLAB’s gradfm_mex)

For masked voxels: uses forward difference where forward neighbor is in mask, falls back to backward difference, or 0 if neither neighbor is in mask. Outside mask: gradient is 0.