NEW YORK — New York Police Department officers trying to get a handle on the spate of gun violence plaguing neighborhoods across the city have run up against a special brand of warfare inflamed by riotous rap lyrics. Rappers getting killed over slights and beefs is sadly nothing new; the industry and music fans are still mourning the East Coast/West Coast-rivalry deaths of Tupac Shakur and the Notorious B.I.G. in the late 1990s. But this strain is something different, investigators say, claiming the lives of talented artists before they barely get a chance to launch their careers. Investigator…