feat(layout): reopen a drawing in the space it was saved in

Files saved while a paper layout was active reopened in Model space
because the loader hardcoded the current layout to "Model" and never
consulted the space recorded in the file.

The active space is a standard part of the format and acadrust already
round-trips it: $TILEMODE (header.show_model_space) for model-vs-paper,
and the CTAB current-tab variable for the exact layout name.

- Read: on open, pick the current layout from CTAB (when it names a
  layout that exists), else $TILEMODE (Model, or the first paper layout).
- Write: Scene::sync_active_space_to_document() mirrors the active layout
  back into the document (show_model_space + CTAB). It runs from
  set_current_layout, so every active-layout change — the status-bar tab,
  the Layout Manager's Set Current, create / rename / delete, and
  undo/redo — keeps the document in sync for the next save.

$TILEMODE is the guaranteed mechanism (read and written for both DXF and
DWG); CTAB refines the exact tab when the variable is present.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/layout.rs

@@ -89,6 +89,7 @@ impl Scene {
         // If the deleted layout was active, fall back to Model space.
         if self.current_layout == name {
             self.current_layout = "Model".to_string();
+            self.sync_active_space_to_document();
         }
 
         self.bump_geometry();

src/scene/mod.rs

@@ -1333,11 +1333,22 @@ impl Scene {
     pub fn set_current_layout(&mut self, name: String) {
         if self.current_layout != name {
             self.current_layout = name;
+            self.sync_active_space_to_document();
             self.recolor_meshes();
             self.bump_geometry();
         }
     }
 
+    /// Mirror the active space (Model tile-mode vs a paper layout) into the
+    /// document's persisted settings so it round-trips on save: the `$TILEMODE`
+    /// header (`show_model_space`) and the `CTAB` current-tab variable. The
+    /// reader restores it via [`current_layout`] on open. Called whenever the
+    /// active layout changes; the file otherwise always reopened in Model.
+    pub fn sync_active_space_to_document(&mut self) {
+        self.document.header.show_model_space = self.current_layout == "Model";
+        crate::io::set_saved_active_layout(&mut self.document, &self.current_layout);
+    }
+
     /// Returns true if this viewport should display model-space content
     /// (i.e. it is a user viewport, not the sheet/overall viewport).
     ///