Function merge_vml_controls 

pub fn merge_vml_controls(
    existing_vml: &[u8],
    controls: &[FormControlConfig],
    start_shape_id: usize,
) -> Vec<u8> 

Merge new form control VML into existing VML bytes (for sheets that already have VML content from comments or other controls).

This appends new shape elements before the closing </xml> tag and updates the shapetype if needed.