Brünnhilde made a guest appearance Friday night in the middle of J.S. Bach's joyous Cantata No. 51 ("Jauchzet Gott!"), a piece usually sung by lyric sopranos of the Kathleen Battle mold. On the word "Alleluja," a remarkable high C came out of the mouth of Barbara Quintiliani and, parting audience members' hair on the way, blazed out of Faneuil Hall into the night sky.That might have just made my day.