Function morphological_bottom_hat 

fn morphological_bottom_hat(
    data: &[f64],
    nx: usize,
    ny: usize,
    nz: usize,
    radius: i32,
) -> Vec<f64>

Morphological bottom-hat filter (closing - original)

Enhances dark features smaller than the structuring element. Uses true spherical structuring element for accuracy. Optimized with pre-computed linear offsets and interior/boundary separation.