Cultural buildings at the site of the World Trade Center are mired in debates about what is “appropriate.” ” ‘The challenge for the curators is going to be: given the context of where these cultural institutions are, what’s appropriate here?’ Whatever else this particular controversy has illustrated, it is just the latest trouble to visit the four cultural structures of the new World Trade Center site. One way or another, the future of all four of them — the memorial, the memorial museum, the performing arts center, and the cultural building — is unclear.