Function build_pyramid 

pub fn build_pyramid<'a>(
    base: ImageView<'a>,
    params: &PyramidParams,
    buffers: &'a mut PyramidBuffers,
) -> Pyramid<'a>

Build a top-down image pyramid using fixed 2x downsampling.

The base image is always included as level 0. Each subsequent level is a 2x downsampled copy (box filter) written into buffers. Construction stops when:

• either dimension would fall below min_size, or
• num_levels is reached.