Function push_stack_member_at 

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

Position-aware variant of push_stack_member: adds block_id to the stack of the leaf holding target_block_id, at position relative to target_block_id instead of always appending. Promotes a single-block leaf (empty block_stack) into a real stack first, exactly like push_stack_member does.