Function pool_window_ready 

pub fn pool_window_ready(
    state: &Arc<AppState>,
    args: &Value,
) -> Result<Value, String>

Phase 6 — frontend signal that a pool window’s renderer is ready to receive pool:promote. Called from awaitPoolPromote AFTER the listener is installed. Only after this signal does the window enter the pool queue (otherwise emit_event_to_window would race the listener install and lose promote events).