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 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -2569,7 +2569,7 @@ impl OpenCADStudio {
                     self.push_undo_snapshot(i, "LAYOUT RENAME");
                     self.tabs[i].scene.rename_layout(&old_name, &new_name);
                     if self.tabs[i].scene.current_layout == old_name {
-                        self.tabs[i].scene.current_layout = new_name.clone();
+                        self.tabs[i].scene.set_current_layout(new_name.clone());
                     }
                     self.layout_manager_selected = new_name.clone();
                     self.tabs[i].dirty = true;
@@ -2610,8 +2610,7 @@ impl OpenCADStudio {
                     self.tabs[i].dirty = true;
                     // Switch to Model if active layout was deleted.
                     if self.tabs[i].scene.current_layout == name {
-                        self.tabs[i].scene.current_layout = "Model".to_string();
-                        self.tabs[i].scene.bump_geometry();
+                        self.tabs[i].scene.set_current_layout("Model".to_string());
                     }
                     self.layout_manager_selected = "Model".to_string();
                     self.layout_manager_rename_buf = String::new();
@@ -2658,8 +2657,7 @@ impl OpenCADStudio {
             Message::LayoutManagerSetCurrent => {
                 let i = self.active_tab;
                 let name = self.layout_manager_selected.clone();
-                self.tabs[i].scene.current_layout = name.clone();
-                self.tabs[i].scene.bump_geometry();
+                self.tabs[i].scene.set_current_layout(name.clone());
                 self.command_line
                     .push_output(&format!("Switched to layout '{name}'."));
                 Task::none()