Function apply_stencil 

fn apply_stencil(
    output: &mut [f32],
    input: &[f32],
    stencil: &[[[f32; 3]; 3]; 3],
    mask: Option<&[u8]>,
    nx: usize,
    ny: usize,
    nz: usize,
)

Apply dipole stencil to a 3D volume Uses Neumann BC at boundaries (matching Julia’s wave_local)

With mask, the output is zero outside it; without, it is computed at every voxel (the reference’s wave_local, zero only on the box boundary).