WASHINGTON — U.S. job growth surged and unemployment fell last month, an unexpected show of strength that may prove costly to homebuyers and businesses who were counting on sharply lower interest rates to lower the cost of buying everything from refrigerators to homes. Employers added 256,000 jobs last month, up from 212,000 in November, the Labor Department reported Friday. Unemployment, which was expected to hover around 4.2%, fell to 4.1% last month.