WASHINGTON — In a major expansion of gun rights after a series of mass shootings, the Supreme Court said Thursday that Americans have a right to carry firearms in public for self-defense, a ruling likely to lead to more people legally armed. The decision came out as Congress and states debate gun-control legislation.About one-quarter of the U.S. population lives in states expected to be affected by the ruling, which struck down a New York gun law. The high court’s first major gun decision in more than a decade …