Keith Glover's "The Rose of Corazon: A Texas Songplay," in its world-premiere run at Shepherdstown, W.Va.'s Contemporary American Theater Festival, marks a refreshing change of pace for a tired genre. Violating the inflexible tenets of theatrical postmodernist dogma, it's an intelligent little musical propelled by understandable, metrical lyrics that embroider attractive, listenable tunes. What a concept.