Over 32 years, Alanna Heiss built P.S. 1 into one of the city’s most refreshingly unpredictable venues for contemporary art, drawing crowds of young, aggressively hip visitors to see its exhibitions and join in its boozy summer dance parties. But when P.S. 1 was merged into the Museum of Modern Art in 2000, it became an open question how long its idiosyncratic impresario would remain at the helm. Now she’s leaving…