Linda Dorcena Forry is the fastest-rising political star of so-called New Boston. She’s the first non-Irishman to host Southie’s vaunted St. Patrick’s Day breakfast. But she earned the inside track by following a familiar playbook: the one written by the city’s Irish bosses of yore. Already, she’s proving that she’s better at Old Boston politics than many Old Bostonians. And some of them have already learned a hard lesson the old-fashioned way.