Layouts reopen in the viewport they were saved in (#1670)

A save records the entered model viewport (or the sheet) as the layout's
last active viewport, and opening the layout enters it again with its
UCS. Picks up the codec fix that kept a last-active link from being read
as the sheet viewport (codec d57097c, kernel b40fa59, graph 2360341).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

tests/active_space_roundtrip.rs

@@ -77,3 +77,44 @@ fn ctab_is_created_when_absent_then_updated_in_place() {
         Some("Layout3")
     );
 }
+
+/// A layout saved with a model viewport entered reopens inside it; saved
+/// from paper space it reopens on the sheet (#1670).
+#[test]
+fn entered_viewport_survives_a_save_and_reload() {
+    use codec::{entities::Viewport, EntityType};
+    for format in ["dxf", "dwg"] {
+        let mut scene = Scene::new();
+        scene.set_current_layout("Layout1".to_string());
+        let mut sheet = Viewport::new();
+        sheet.id = 1;
+        sheet.width = 420.0;
+        sheet.height = 297.0;
+        let sheet = scene.add_entity(EntityType::Viewport(sheet));
+        let mut model = Viewport::new();
+        model.id = 2;
+        model.width = 100.0;
+        model.height = 80.0;
+        model.view_height = 80.0;
+        let model = scene.add_entity(EntityType::Viewport(model));
+        assert_ne!(sheet, model);
+
+        for entered in [Some(model), None] {
+            scene.active_viewport = entered;
+            scene.sync_active_viewport_to_document();
+            let bytes =
+                OpenCADStudio::io::save_to_bytes(&scene.document, format, scene.document.version)
+                    .expect("save");
+            let doc = OpenCADStudio::io::load_bytes(&format!("roundtrip.{format}"), bytes)
+                .expect("reload");
+            let mut reopened = Scene::new();
+            reopened.document = doc;
+            reopened.current_layout = "Layout1".to_string();
+            assert_eq!(
+                reopened.saved_entered_viewport().is_some(),
+                entered.is_some(),
+                "{format}: entered {entered:?}"
+            );
+        }
+    }
+}