China’s leaders have long known that the country is nearing a demographic crossroads. Policymakers have warned that China must prepare for a slowly shrinking population and an era of fewer workers and more retirees. State media have urged young couples to seize the opportunity to have two or three children under relaxed family-size rules, to soften the looming economic crunch. And yet the sense of incipient crisis grew on Tuesday, when the government confirmed that the nation’s population shrank last year for the first time in six decades, sooner and…