With one missile fired over the Pacific Ocean from a submarine lurking off China’s coast, the country’s leader, Xi Jinping, has proclaimed his determination to build a full suite of nuclear weapons, including sea-based missiles that have long been Beijing’s weak spot. The missile test on Monday came after China’s military has been battered for years by mass removals of commanders accused of corruption and disloyalty. But Chinese media declared that the test showed the country remained on track to create a full nuclear triad; that is, a range of…