WASHINGTON — The U.S. Supreme Court, the setting for landmark rulings in favor of free speech, can keep protesters off its marble plaza without violating their constitutional rights, a federal appeals court ruled today.The inviting 20,000-square-foot, open-air plaza can remain a protest-free zone because the court has an interest in preserving decorum and the idea that judges are not influenced by public opinion and pressure, said a unanimous three-judge panel of the U.S. Court of Appeals for the District of Columbia …