Function split_vertical 

pub fn split_vertical(
    root: &mut LayoutNode,
    target_id: &str,
    new_node: LayoutNode,
    position: SplitPosition,
) -> Result<(), LayoutError>

Vertical split: insert new_node before/after target_id.

• If the target’s parent is a Column, splice directly.
• Otherwise, wrap in a fresh Column group.