Refraction AI calls itself the Goldilocks of robotic delivery. The Ann Arbor-based company, which recently raised a $4.2 million seed round and expanded operations to Austin, was founded by two University of Michigan professors who think delivery via full-size autonomous vehicles (AV) is not nearly as close as many promise and sidewalk delivery comes with too many hassles and not enough payoff. Their “just right” solution? Find a middle path, or rather, a bike path.