If Austin scores Amazon’s $5 billion second headquarters, we should brace ourselves for a hotter — and costlier — market for single-family homes. A new report from Amherst Capital Management LLC, a real estate investment firm, forecasts demand for single-family homes in the Austin metro area would jump by 13 percent if Amazon picks our region for the e-commerce giant’s so-called HQ2 project. By comparison, demand would inch up by just 4 percent if Dallas gains HQ2.