Function apply_stencil 

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

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