A class-action lawsuit about overtime pay for truck drivers hinged entirely on a debate that has bitterly divided friends, families and foes: The dreaded — or totally necessary — Oxford comma, perhaps the most polarizing of punctuation marks. What ensued in The United States Court of Appeals for the First Circuit, and in a 29-page court decision handed down on Monday, was an exercise in high-stakes grammar pedantry that could cost a dairy company in Portland, Me., an estimated $10 million.