The Department of Homeland Security, positioning itself to cut its losses on a so-called invisible fence along the U.S.-Mexico border, has decided not to exercise a one-year option for Boeing to continue work on the troubled multibillion-dollar project involving high-tech cameras, radar and vibration sensors. The result, after an investment of more than $1 billion, may be a system with only 53 miles of unreliable coverage along the nearly 2,000-mile border.