DETROIT — For more than a year, a lawsuit filed by Brooke Melton’s family has caused major migraines for General Motors. Litigation over the 29-year-old nurse’s death was settled by GM in October. But not before it laid bare how the company allowed millions of small cars to stay on the road more than a decade after GM discovered ignition switch flaws linked to at least 13 fatalities. Now the case could upend GM’s strategy to compensate victims and limit its legal liabilities in such tragedies …