Anxiety about Greece may keep Wall Street on edge early in the week, as the country moves toward what was once thought unthinkable: a default and a full exit from the euro zone. While European markets are braced for a wave of contagion from Greece on Monday, as a result of a possible “Grexit” that most had still assumed was unlikely as late as Friday afternoon, the effect on U.S. markets is expected to be bumpy, but not as dramatic as in Europe. Athens is due to miss a 1.6 billion euro ($1.8 billion) repayment to the International Monetary Fund on Tuesday.