Subprime lenders have won a major concession from federal regulators seeking to combat predatory lending: second mortgages are largely spared from the Federal Reserve Board’s expanded high-cost mortgage regime. In a long-awaited final rule approved this month, the Fed cut, to 8 percent above comparable Treasury securities, the interest-rate trigger that defines a mortgage as high-cost under the Home Ownership and Equity Protection Act. But in a shift from the agency’s proposed rule issued a