A truck bomb Saturday caused a fire and the collapse of a section of a bridge linking Russia-annexed Crimea with Russia, Russian officials say, damaging a key supply artery for Moscow’s faltering war effort in southern Ukraine. The speaker of Crimea’s Kremlin-backed regional parliament immediately accused Ukraine, though the Kremlin didn’t apportion blame. Ukrainian officials have repeatedly threatened to strike the bridge and some lauded the attack, but Kyiv stopped short of claiming responsibility. The bombing came a day afterRussian President Vladimir Putin turned 70, dealing him a humiliat…