Eric Geoffroy calls it “the delicate princess of doughnuts.’’ Skye Gaudette once sketched a picture of it on a napkin in her quest to describe it. Wendy Cobrda wrote to the company for an explanation when it vanished from her local coffee shop. They are devotees of the French Cruller. And pursuing their obsession has not been an easy path. The French Cruller, with its distinctive twisted ridges, disappeared three years ago from a broad corridor along Route 3.