For a variety of oh-so-obvious reasons, project managers of the new tower to rise above the site of the World Trade Center say they’re building the “safest building in the world.” “In an attempt to live up to that very public promise — to overcome public fear, and reassure prospective tenants — the designers of the tower are carrying out a most unusual exercise that is in equal parts brainstorming, forensic analysis and Götterdämmerung-style what-iffing.”