LE BOURGET, France (AP) — Young supporters wore his name on their backs like a soccer star’s, above the number 27. They sang about brass bands and revolution. Many had spent the fall protesting or blockading their schools. Then in walked Jean-Luc Mélenchon: 75 years old, on his fourth presidential run and old enough to be their grandfather. “I have only one sensible thing to propose,” he told 20,000 supporters in a vast hall north of Paris on Saturday. “Everything must change.” The septuagenarian may never get another chance. Three presidential bids have ended short of the runoff. In 2022,...