Two giants of twentieth-century opera launching a new star for the 21st: That is just part of what happened in this new production that celebrated and made Miami music history. The first collaboration between the great Renata Scotto and Richard Bonynge, this local staging of Bellini's La Sonnambula was a demanding and gorgeous romantic opera. Starring Miami's favorite young diva, Leah Partridge, in a role she seemed born to sing, La Sonnambula was opera heaven, a thrilling and impossibly beautiful show for newcomers and jaded operamaniacs alike. One of the sweetest things about it was that, like the the American Ballet Theatre's Swan Lake, the opera was the kind of production that could not have happened in Miami before the Carnival Center's opening. The Florida Grand Opera, one of the prime movers behind getting the center built in the first place, deserves this new Ziff Opera House, with its ideal acoustics and against-all-odds intimate atmosphere. What's the big deal about operaç The Florida Grand Opera, when it is this good, is a good answer.