"Ah, this is ballet," sighed one white-haired woman to another. And then, for emphasis, "This is ballet." Originally choreographed by George Balanchine, this is Francia Russell's staging of the master's A Midsummer Night's Dream (at McCaw Hall through April 13, tickets $20-$150). It's "real" ballet in the way that a Cheever short story signifies the New Yorker. But we're not here to beat up on oldsters, according to our parole officer--and neither is the Peter......