Layouts: first opening follows the page setup; viewports picked by their frame

- A layout opens for the first time when its page setup was never
  initialized (no paper size, no media name) or it was created in this
  session; its page setup is initialized then, and with
  LAYOUTCREATEVIEWPORT it gets its first viewport. A layout never opened
  is saved with an uninitialized page setup and no viewports, so it gets
  its viewport when the file is opened again, here or in the reference.
- Select Viewport for view: a viewport is selected when the pick box
  (PICKBOX pixels at the sheet's zoom) touches its frame; a pick well
  inside or outside selects nothing and the prompt repeats.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scene/entity.rs

@@ -3098,6 +3098,7 @@ impl Scene {
             if let codec::objects::ObjectType::Layout(l) = obj {
                 if l.name != "Model" {
                     crate::scene::apply_default_page_setup(l, "");
+                    self.unopened_layouts.insert(l.name.clone());
                 }
             }
         }