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/viewport.rs

@@ -3050,7 +3050,7 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                                 }
                             }
                         }
-                        self.tabs[i].scene.current_layout = new_name.clone();
+                        self.tabs[i].scene.set_current_layout(new_name.clone());
                         // Safety net — `add_layout` already creates the overall
                         // sheet viewport; this covers any path that doesn't.
                         self.tabs[i].scene.ensure_sheet_viewport(&new_name);
@@ -3085,7 +3085,7 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                             self.push_undo_snapshot(i, "LAYOUT RENAME");
                             self.tabs[i].scene.rename_layout(&orig, &new_name);
                             if self.tabs[i].scene.current_layout == orig {
-                                self.tabs[i].scene.current_layout = new_name.clone();
+                                self.tabs[i].scene.set_current_layout(new_name.clone());
                             }
                             self.tabs[i].dirty = true;
                             self.command_line