Short-lived governments are routine in some democracies, with uncertainty the norm and predictability ever elusive. Yet never in London, where hundreds of years of steady parliamentary rule set a standard for the world going back to the first prime minister, Robert Walpole, three centuries ago. His two decades were a picture of stability. Not now, as His Majesty’s government is in chaos, with Prime Minister Liz Truss out after a mere six-week stay at 10 Downing St., besting even the brevity of the forgotten premiership of George Canning, who died after only four months when John Quincy Adams w…