A construction worker jumped to his death from a ninth-floor window this morning at a Midtown building — his body plummeting to a lower-level rooftop as a dentist working on another floor watched in horror, authorities said. “It was like nothing I’ve ever seen,” the dentist told The Post, saying he was in his 7th floor office at 49 W. 57th St., working on a patient, when Christopher Cutrone, 32, fell past his window at 10:08 a.m.