The opera is on in Rappahannock County beginning tomorrow with a 7 p.m. performance of “The Turn of the Screw.“  The first Castleton Festival—believed to be the only chamber-opera festival in the country—kicks off at the farm of Lorin Maazel, the recently retired director of the New York Philharmonic, and his wife, actress Dietlinde Turban L. MaazelMaazel. It’ll run through July 19, with performances most days.