SEOUL, South Korea — North Korean scientists have learned to produce crucial components of gas centrifuges inside their isolated country, undermining years of export controls and sanctions intended to stop the country’s enrichment of uranium for nuclear weapons, according to an analysis by two American arms control experts made available on Monday. Multimedia Timeline Timeline of North Korea’s Nuclear Program Connect With Us on Twitter Follow @nytimesworld for international breaking news and headlines. Twitter List: Reporters and Editors The analysis comes as experts have reported other signs that North Korea is activating or expanding its nuclear production facilities. Taken together, they…