About a year from now, Anne Teresa De Keersmaeker will reach a mainstream audience when she stages the dances for Ivo van Hove's Broadway production of West Side Story.  Both she and her company Rosas frequent BAM, various Lincoln Center festivals and Baryshnikov Arts Center, which over this long weekend presented the New York premiere of the Belgian modernist's stunning Verklärte Nacht in the Jerome Robbins Theater.  There's no irony intended here.  It's BAC's largest performing space and the two choreographers are generationally, aesthetically and fundamentally different from each other. Verklärte Nacht (Transfigured Night), originally choreographed in 1995 and reconstructed in 2014, is based