Function write_group_modular_section 

pub fn write_group_modular_section(
    group_image: &ModularImage,
    state: &GlobalModularState,
    writer: &mut BitWriter,
) -> Result<()>

Writes a group’s data section for multi-group modular encoding.

This writes:

• GroupHeader (use_global_tree=1, wp_header.all_default=1, num_transforms=0)
• Encoded pixel residuals using HybridUint {4,2,0} + global entropy codes

The group_image should be the extracted region for this group.