The strange disappearance from public view of China’s presumptive new leader is turning a year that was supposed to showcase the Communist Party’s stability into something of an annus horribilis. Over the past week, the new leader, Xi Jinping, has missed at least three scheduled meetings with foreign dignitaries, including Secretary of State Hillary Rodham Clinton last Wednesday and the prime minister of Denmark on Monday. So far officials have declined to provide an explanation for his absences.