LONDON — Britain’s embattled Keir Starmer stared down one leadership rival Thursday lunchtime. Another was banging on his door by nightfall. A putsch to oust the center-left prime minister by competing factions within his own party is finally taking shape after the most turbulent week in U.K. politics since former Conservative PM Liz Truss resigned weeks into the job in 2022. Deep election losses last Thursday led 96 of Starmer’s 402 Labour MPs to demand a timeline for his exit. By refusing to