SINGAPORE — When President Trump and North Korean leader Kim Jong Un shake hands for the first time and sit down one-on-one here Tuesday, the bonhomie they plan to project will mask the huge gulf between their two countries as diplomats struggle to broker a deal for the rogue state to abandon its nuclear weapons. The decision by Trump and Kim to begin their Singapore summit without their top advisers or nuclear arms specialists in the room underscores that their real goal is to develop a rapport and stage a global spectacle rather than to ink the technical details of a denuclearization accord. Both nations sought to lower expectations for an immediate breakthrough this week in Singapore.