Four days after US President Donald Trump departed Beijing, Russian President Vladimir Putin arrived — a swift diplomatic handover that underscored China’s central place in an increasingly unsettled global order. Beijing’s ties with Washington and Moscow differ sharply, of course: one relationship is defined by strategic rivalry, the other by strategic partnership. Yet as China welcomes a steady procession of global leaders, each high-profile visit helps reinforce the sense that Beijing is becoming a pivotal stage for world…