OSAN AIR BASE, South Korea — The United States on Tuesday sent nuclear-capable supersonic bombers streaking over ally South Korea in a show of force meant to cow North Korea after its recent nuclear test and also to settle rattled nerves in the South. The B-1B bombers, escorted by U.S. and South Korean jets, were seen by an Associated Press photographer as they flew over Osan Air Base, which is 75 miles from the border with North Korea, the world’s most heavily armed.