ANKARA, Turkey — President Lee Jae Myung left his first NATO summit with both a vision and a vehicle to advance it: a proposal to move South Korea beyond the role of an arms exporter toward deeper defense industrial cooperation with the alliance, and the launch of negotiations on an agreement that would help make that shift possible. At the summit in Ankara, Lee called for South Korea and NATO to move toward joint development, production and operation of weapons systems through a “Korea-NATO Def