KYIV, Ukraine — Rescuers began to pull survivors Thursday from the rubble of a nighttime airstrike on a theater where more than 1,000 people had sheltered in the besieged port city of Mariupol, as Russia’s war on Ukraine entered its fourth week with no sign of a respite. The death toll at the red-roofed building in Mariupol, which satellite images showed had the word “children” written on the…