It takes a special combination of skills to run the Orpheum Theatre. Raising more than a million dollars every year is one of them. Being a risk-taker is another. For 35 years, Pat Halloran made an enormous impact on the institution. Along with restoring and expanding the building, he helped turn regional theaters like the Orpheum into New York producers, giving them leverage in an industry once dominated by a small coterie of Broadway investors. When Halloran steps down next week, he will leave both a proud legacy and a formidable challenge.