Now that the New Museum on Manhattan’s Lower East Side is complete, and its structure enclosed, there is little evidence of the system that supports the seven-story building that seems to be made up of nothing heavier than precariously stacked cardboard boxes. But here and there, through its expanded metal-mesh facade, and from behind windows, architects Kazuyo Sejima and Ryue Nishizawa of Tokyo-based SANAA have provided an occasional glimpse of a diagonal brace.