layoutInDocument function
host's layout (frame, obstacles, stretches) through the document
adapter (wallsInDocument, with moved standing in for the walls it
names). Null when host is not a live wall or is degenerate.
Implementation
HostLayout? layoutInDocument(DraftDocument doc, Handle host,
{Map<Handle, WorldWall> moved = const {}}) {
final w = wallsInDocument(doc, host, moved: moved);
return w == null ? null : layoutOf(w.host, w.walls);
}