Sarah Kirkland Snider's first opera arrives with a confidence that feels almost paradoxical: it is at once tightly focused and lavishly expansive, a work that fixes its gaze on a single hinge in medieval history while allowing the implications of that moment to ripple outward in all directions. "Hildegard" does not so much resurrect Hildegard von Bingen as acknowledge what she has always seemed to be"a figure who bel…
