The choreographer Lucinda Childs is having a moment. On June 26, the day she turns eighty-six, she’s opening a weekend run of “Momentary Reprise,” a program featuring new work, at Bard SummerScape (running at the Fisher Center, in Annandale-on-Hudson, N.Y., through June 28). She’s also performing in it. This comes amid a flurry of activity rare for an octogenarian. A few months ago, Childs had a r...