SEOUL, South Korea — North Korea said Friday it has christened a purported nuclear attack submarine it has been developing for years, a step leader Kim Jong Un described as crucial in his efforts to build a nuclear-armed navy to counter the United States and its Asian allies. The North’s official Korean Central News Agency said the vessel — named “Hero Kim Kun Ok” — is designed to launch tactical nuclear weapons from underwater but did not specify the number of missiles it could carry and fire. In speeches at the vessel’s launching ceremony on Wednesday and an onboard inspection…