Condominium booms are like Cher: Every time you think they’re finished, another comeback is around the corner. Despite the sluggish national economy and 7.8 percent unemployment, the rising stock market and the return of Wall Street bonuses have once again stoked demand for the shiny-and-new. Because few buildings were begun in 2009 and 2010, when financing was nearly impossible, such properties are in short supply. Those factors, according to Streeteasy.com, drove the median Manhattan price for new condos up by 16.7 percent, to $1.575 million, in the past twelve months, with the number of signed contracts up by a third. Unlike the behemoths of the previous boom, many of these are smaller buildings (fewer than 100 units) yet are still high-end, with big apartments and luxury finishes. Because banks were leery about lending during the crash, developers who did get approved were on solid footing: If you’re buying from floor plans, the odds of avoiding a debacle are probably better than they were pre-Lehman.