Function convolve_y 

pub(crate) fn convolve_y(
    src: &Pixmap,
    dst: &mut Pixmap,
    src_width: u16,
    src_height: u16,
    kernel: &[f32],
    radius: u8,
    edge_mode: EdgeMode,
)

Apply vertical blur pass (1D convolution along y-axis).

For each output pixel, computes a weighted sum of vertically neighboring pixels using the Gaussian kernel. Handles edge cases according to the specified edge mode. Writes results to a destination buffer to avoid overwriting source data.