Function reorder_within_leaf 

fn reorder_within_leaf(
    tree: &mut LayoutNode,
    block_id: &str,
    target_block_id: &str,
    position: StackMovePosition,
    activate: bool,
) -> bool

Splice block_id to position relative to target_block_id within the SAME leaf’s stack. Both must already be members of that leaf (checked by the caller, move_stack_member). Only touches block_id/active_block_id when activate is set — a reorder alone never changes which member is visible.