SINCE TRISHA BROWN’S ANNOUNCEMENT, early last year, that she would step down from the helm of her company, the troupe has been on an international tour. It is not a farewell but, rather, a celebration: Diane Madden and Carolyn Lucas, two longtime company members, will take the reins and continue presenting Brown’s dances. This summer, on Governors Island, the River to River Festival honors Brown’s relationship to New York City, presenting exhibitions, talks, and open rehearsals devoted to her role as one of the earliest practitioners of site-specific dance. Brown, whose works range from pure dance to pure concept, has asked her troupe to perform on roofs, on walls, and on rafts. In 2011, she created three final dances, including “I’m Going to Toss My Arms—If You Catch Them They’re…