He had worked the St. Patrick’s Day parade as a cop and now Raymond Kelly was the police commissioner, heading down Fifth Ave. where he soon would be marching as this year’s grand marshal. Kelly was first expected at the preparade Mass at St. Patrick’s Cathedral. He was reaching E. 84th St. comfortably on schedule when a bicyclist struck a woman directly ahead. “Hit her very hard,” he later said. Suddenly, Kelly was a cop again. He jumped out of the black commissioner’s SUV in a formal morning coat and grand marshal’s sash with the same impulse to help that had propelled him from a radio car countless times.