TO BE BLUNT, there’s opera, and then there’s Wagner. By the time Richard Wagner had completed Parsifal in 1882, he was using the word bühnenweihfestspiel (“festival play for the consecration of a stage”) instead of “opera” to describe this four-and-a-half-hour epic, where music, drama, lighting, architecture, and quasi-religious ritual come together to create what the Germans called “gesamtkunstwerk,” or a total work of art. In the past decade, only two U.S. opera houses have had the guts to take on Parsifal, which makes the upcoming Houston Grand Opera production even more of a must-see, given how rarely this complex and controversial opera is staged.