Function dipole_kernel_f32 

pub fn dipole_kernel_f32(
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f32,
    vsy: f32,
    vsz: f32,
    bdir: (f32, f32, f32),
) -> Vec<f32>

Generate dipole kernel in k-space (f32 version for WASM performance)

Creates the dipole kernel D(k) = 1/3 - (kz)²/|k|² (for B = [0,0,1]) centered at index (0, 0, 0) (not shifted).