DETROIT — Detroit is eligible to shed billions in debt in the largest public bankruptcy in U.S. history, a judge said today in a long-awaited decision that now shifts the case toward how the city will accomplish that task. Judge Steven Rhodes turned down objections from unions, pension funds and retirees, which, like other creditors, could lose under any plan to solve $18 billion in long-term liabilities. But that plan isn’t on the judge’s desk yet. The issue for Rhodes, who presided over a nine …