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 <[email protected]>

src/scene/parametric_constraints.rs

@@ -2535,6 +2535,9 @@ impl super::Scene {
         }
         crate::io::xref::strip_resolved_xref_content(&mut document);
         crate::entities::field::stamp_save_dates(&mut document);
+        for name in &self.unopened_layouts {
+            crate::scene::write_layout_unopened(&mut document, name);
+        }
         document
     }