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/file.rs

@@ -489,7 +489,28 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 }
                 self.tabs[i].scene.selected = rustc_hash::FxHashSet::default();
                 self.tabs[i].scene.preview_wires = vec![];
-                self.tabs[i].scene.current_layout = "Model".to_string();
+                // Reopen in whichever space the file was saved in — the CTAB
+                // tab name when recorded, else the $TILEMODE model/paper flag —
+                // instead of always landing in Model.
+                {
+                    let names = self.tabs[i].scene.layout_names();
+                    let saved = crate::io::saved_active_layout(&self.tabs[i].scene.document)
+                        .and_then(|n| {
+                            names.iter().find(|x| x.eq_ignore_ascii_case(&n)).cloned()
+                        });
+                    self.tabs[i].scene.current_layout = match saved {
+                        Some(n) => n,
+                        None if self.tabs[i].scene.document.header.show_model_space => {
+                            "Model".to_string()
+                        }
+                        // Paper was active but no CTAB — fall back to the first
+                        // paper layout (names[0] is always "Model").
+                        None => names
+                            .into_iter()
+                            .nth(1)
+                            .unwrap_or_else(|| "Model".to_string()),
+                    };
+                }
                 // Rebuild the Isolate/Hide set from the entities the file itself
                 // marks invisible (DXF code 60), so hidden objects stay hidden on
                 // reopen and End Isolation can bring them back.