KYIV, Ukraine — She had walked for hours through the Lyman grey zone, past shell craters and the bodies of neighbors who hadn’t made it out, when the robot caught up to her. The 77-year-old saw it first as a blanket, then as the three words painted across it in an operator’s hand: “Grandma, get on!” Ukraine’s 3rd Army Corps and its Cerberus unmanned ground systems unit ran the April 25 rescue with...