Function apply_monitor_topology_changed 

pub fn apply_monitor_topology_changed(
    state: &mut State,
    rects: Vec<Rect>,
) -> Vec<Event>

Phase B.9.1 — handle Command::ReportMonitorTopologyChanged. Replaces state.monitors wholesale, then re-evaluates every known mirror’s last_rect against the new set. Emits OffMonitor for any window that newly falls off (e.g. user unplugged the external display where the window lived).