Martin Dunn, 38, for example, is putting up housing aimed at the bottom rungs of the economic ladder: limited-income working families and people with psychiatric disabilities, many of them homeless. And he is doing so through a profit-making venture.What he builds is called supportive housing, and he has completed two midrise apartment houses with 130 units and has started or is planning 11 more projects, all of them in Brooklyn or the Bronx.