In a historic turn, China’s rise as an economic superpower is reversing. The biggest global story of the past half century may be over.     After stagnating under Mao Zedong in the 1960s and 70s, China opened to the world in the 1980s — and took off in subsequent decades. Its share of the global economy rose nearly tenfold from below 2 per cent in 1990 to 18.4 per cent in 2021. No nation had ever risen so far, so fast.    Then the reversal began. In 2022, China’s share of the world economy shrank a bit. This year it will shrink more significantly, to 17 per cent. That two-year drop of 1.4 per cent is the largest since the 1960s.