At lunchtime on Thursday, my phone pinged. It was a video message from a Labour source, a gif of a shadowy hand flicking over one domino, which knocked over another, then another, then another. Then hundreds, then thousands, came tumbling down. It was obvious what my contact was suggesting, half an hour or so after the shock resignation of John Healey. Could the exit of the now former defence secretary set off a chain of events that would lead Sir Keir Starmer's operation to fall over too? The