TOKYO (AP) — North Korea's latest nuclear test was part theater, part propaganda and maybe even part fake. But experts say it was also a major display of something very real: Pyongyang's mastery of much of the know-how it needs to reach its goal of becoming a full-fledged nuclear state. It remains unclear whether North Korea tested, as it claims, a hydrogen bomb ready to be mounted on an ICBM. But Sunday's test, the sixth and most powerful North Korea has conducted since its first in 2006, was a stunning advance in its demonstrated ability to build high-yield nuclear weapons.