The average long-term U.S. mortgage rate declined again this week, easing borrowing costs for prospective homebuyers during what is typically the housing market’s busiest time of the year. The benchmark 30-year fixed rate mortgage rate dropped to 6.3% from 6.37% last week, mortgage buyer Freddie Mac said Thursday. One year ago, the rate averaged 6.83%. The average rate is now at its lowest level since March 19, when it was 6.22%. Meanwhile, borrowing costs on 15-year fixed-rate mortgages,