A big power company accused of spreading smog and acid rain across a dozen states agreed Tuesday to pay at least $4.6 billion to cut chemical emissions in what the government called the nation’s largest environmental settlement. The agreement with American Electric Power ends an eight-year legal battle over reducing smokestack pollution that drifted across Northeast and mid-Atlantic states, chewing away at mountain ranges, bays and national landmarks. AEP, based on Columbus, Ohio, maintains it never violated Clean Air Act rules to curb emissions.