Function downscale_y 

fn downscale_y(
    src: &mut Pixmap,
    src_width: u16,
    src_height: u16,
    dst_height: u16,
    edge_mode: EdgeMode,
)

Vertical decimation pass using [1,3,3,1]/8 filter.

Reduces logical height by 2x while applying a binomial blur kernel. Operates in-place by writing to the beginning of the same buffer.