Function apply_magnified_node_changed 

fn apply_magnified_node_changed(
    wstore: &WaveStore,
    tab_id: &str,
    node_id: &str,
) -> Result<(), Box<dyn Error>>

Phase E.4 (Option A) — write the reducer-emitted magnified node id onto the tab’s LayoutState.magnifiednodeid column. Same shape as apply_focused_node_changed.