Juan Guaidó, the 35-year-old opposition leader recognized by Washington as Venezuela’s rightful leader, laid out a plan Thursday to reverse President Nicolás Maduro’s economic polices and address Latin America’s worst humanitarian crisis in decades. Speaking in an auditorium packed with supporters at a Venezuelan university, Mr. Guaidó said a new government could quickly stabilize an economy that was once one the region’s richest but that now suffers from widespread food and medical shortages and the world’s highest…