It's a sad fact of the even sadder world we live in that J.T. Rogers's play Oslo, which just opened at the Vivian Beaumont at Lincoln Center following a run downstairs at the Mitzi E. Newhouse Theater last summer, will never be finished. Its specific subject, the 1993 Oslo Accords that led to a famous Rose Garden handshake between Israeli Prime Minister Yitzhak Rabin and Yasser Arafat, chairman of the Palestine Liberation Organization (PLO), would seem to have a definitive (if not always satisfying) beginning, middle, and end. But anyone who pays attention knows that the concept of peace in the Middle East is equal part dream and nightmare, and usually dissolves as quickly as it appears, in light that can be far less bright than that of the morning sun. So where is there for such a play to go? . . .