The Federal Reserve’s single largest intervention to prop up the American economy, its $1.25trn program to buy mortgage-backed securities, came to a long-anticipated end on Wednesday. The program has been credited with holding mortgage interest rates at near-record lows and slowing the nationwide decline in home prices that threatened to send the economy into an extended slump. When the central bank announced the program two days before Thanksgiving 2008, the spread, or difference, between the rates for a 30-year fixed-rate mortgage and a 10-year Treasury note exceeded 2.5 percentage points, or 250 basis points, nearly twice the typical spread.