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/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();