KYIV — The terrible cost of Russia’s continuing assault on Ukraine is viscerally clear at a military rehabilitation center on the outskirts of this city. Soldiers there describe how their bodies were shattered on the front lines. And they’re the lucky ones who survived. Alexei was trying to hold his position at Pokrovsk, the scene of some of this year’s heaviest fighting, when a drone dropped a grenade near him. His left leg and right hand were nearly severed, attached by thin threads of tissue but now mended. Nikolai lost his left leg in Kharkiv, another Russian target. He waited...