Leonard Bernstein would have turned 90 years old last Monday, and next month Carnegie Hall and the New York Philharmonic will join forces to celebrate the occasion with "The Best of All Possible Worlds," a four-month festival of performances, lectures, film screenings, and general hoopla. Truth to tell, the festivities have already started -- PBS has been replaying its 1998 "American Masters" documentary on Bernstein at what seems like hourly intervals, and summer music festivals are programming his compositions by the carload -- so by the time Michael Tilson Thomas and the San Francisco Symphony finally get around to a pair of all-Bernstein concerts in San Francisco on Sept. 17 and kick [...]