New market entrants and strong borrower demand fueled a surge in Alt-A lending during the first half of the year, setting a new record that far exceeded volume in the rival agency-backed market and the mortgage market as a whole. Overall, Inside Mortgage Finance estimates that $90 billion in new Alt-A loans were generated during the first six months of this year, a 136.8 percent rise from the same period in 2003, when Alt-A volume…