On Tisha B'Av, the saddest day in the Jewish calendar marking the destruction of the Temple in Jerusalem and the long history of catastrophes that followed, an assailant armed with a knife walked up to two men on Manhattan's Upper West Side and stabbed them. One wore a yarmulke.Witnesses said the assailant shouted, "Allahu Akbar." The victims, ages 50 and 57, survived. A suspect is in custody, and police are examining the motive. But New York City did not need to wait for a formal designation to understand the gravity of the crime that had been perpetrated.Only a day earlier,