Yoga is as ancient as Sanskrit and as American as Walden Pond — Thoreau proclaimed “even I am a yogi” in 1849. Modern yoga is as rooted in the nation’s capital as Dumbarton Oaks in Georgetown, where the marvelously named philanthropist Mildred Bliss performed yoga postures in her famous garden, guided by a personal trainer imported from Austria in the 1930s. Today images of yoga bodies are as mainstream, and as annoying, as the parking tickets illustrated with “calming” poses issued last year in Cambridge, Mass., as part of a public art project.