Function push_stack_member 

pub fn push_stack_member(
    tree: &mut LayoutNode,
    target_block_id: &str,
    block_id: &str,
    activate: bool,
) -> bool

Add block_id to the tab stack of the pane holding target_block_id (as its visible tab or a background member). A pane with no stack yet becomes a two-member stack. Already a member → only (re)activated if asked. Returns false when no pane holds target_block_id. SPEC_PANE_TABS_REDUCER_COMMANDS_2026_09_18.md §3.2 (LayoutStackPush).