fix(layout): create the CTAB variable so the exact paper tab round-trips

Saving from a non-first paper layout still reopened on the first paper
tab: set_saved_active_layout only updated an existing CTAB entry, but
documents authored here never carried one, so CTAB was never written and
the reader fell back to $TILEMODE (which only records model-vs-paper) →
the first paper layout.

set_saved_active_layout now creates the CTAB DICTIONARYVAR under the root
named-object dictionary when it is absent (updating in place otherwise).
The root dictionary is taken from the header handle, or found by scanning
for the dictionary that holds ACAD_LAYOUT when a from-scratch document
has not populated the header handle yet. The writers persist it (they
serialize the document's objects and root-dictionary entries; the
root-dict rebuild in CadDocument::build() is not run on save).

Adds tests/active_space_roundtrip.rs, including a full DXF save→reload
round-trip asserting both $TILEMODE and CTAB survive.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/io/mod.rs

@@ -538,10 +538,47 @@ pub fn saved_active_layout(doc: &CadDocument) -> Option<String> {
 }
 
 /// Record `name` as the active layout tab (`CTAB`) so the next save round-trips
-/// which space was open. No-op when the document has no `CTAB` entry yet; the
-/// `$TILEMODE` header (model vs paper) remains the guaranteed fallback.
+/// which space was open. Updates the existing `CTAB` variable in place, or
+/// creates one under the root named-object dictionary when the drawing never
+/// carried it (e.g. a document authored here from scratch) — otherwise the exact
+/// paper layout would be lost and reopening fell back to the first paper tab.
 pub fn set_saved_active_layout(doc: &mut CadDocument, name: &str) {
-    set_vardict_value(doc, "CTAB", name);
+    use acadrust::objects::{DictionaryVariable, ObjectType};
+    if let Some(h) = vardict_handle(doc, "CTAB") {
+        if let Some(ObjectType::DictionaryVariable(v)) = doc.objects.get_mut(&h) {
+            v.value = name.to_string();
+        }
+        return;
+    }
+    // Attach a new CTAB entry to the root named-object dictionary. Prefer the
+    // header handle; fall back to scanning for the root dict (the one holding
+    // `ACAD_LAYOUT`) since a from-scratch document leaves the header handle null
+    // until it is built. No root dict at all → `$TILEMODE` still records
+    // model-vs-paper.
+    let named = doc.header.named_objects_dict_handle;
+    let root = if !named.is_null() && doc.objects.contains_key(&named) {
+        named
+    } else {
+        match doc.objects.iter().find_map(|(h, o)| match o {
+            ObjectType::Dictionary(d)
+                if d.entries.iter().any(|(k, _)| k.eq_ignore_ascii_case("ACAD_LAYOUT")) =>
+            {
+                Some(*h)
+            }
+            _ => None,
+        }) {
+            Some(h) => h,
+            None => return,
+        }
+    };
+    let handle = doc.allocate_handle();
+    let mut var = DictionaryVariable::new("CTAB", name);
+    var.handle = handle;
+    var.owner_handle = root;
+    doc.objects.insert(handle, ObjectType::DictionaryVariable(var));
+    if let Some(ObjectType::Dictionary(rd)) = doc.objects.get_mut(&root) {
+        rd.entries.push(("CTAB".to_string(), handle));
+    }
 }
 
 /// Materialise the current-style choices into their format-specific storage

tests/active_space_roundtrip.rs

@@ -0,0 +1,79 @@
+// The active space (Model vs a paper layout) a drawing was saved in must
+// round-trip: `set_current_layout` mirrors it into the document as $TILEMODE
+// (`header.show_model_space`) and the CTAB current-tab variable, and the loader
+// restores it. Regression guard for the "always reopens in Model / the first
+// paper layout" bugs.
+
+use OpenCADStudio::scene::Scene;
+
+#[test]
+fn switching_to_paper_records_tilemode_and_ctab() {
+    let mut scene = Scene::new();
+
+    // A paper layout is active → TILEMODE says paper, CTAB names the tab.
+    scene.set_current_layout("Layout1".to_string());
+    assert!(
+        !scene.document.header.show_model_space,
+        "$TILEMODE should record paper space when a layout is active"
+    );
+    assert_eq!(
+        OpenCADStudio::io::saved_active_layout(&scene.document).as_deref(),
+        Some("Layout1"),
+        "CTAB must be created/updated so the exact paper tab round-trips (not \
+         just the first paper layout)"
+    );
+
+    // Back to Model → TILEMODE flips, CTAB follows.
+    scene.set_current_layout("Model".to_string());
+    assert!(
+        scene.document.header.show_model_space,
+        "$TILEMODE should record model space in the Model tab"
+    );
+    assert_eq!(
+        OpenCADStudio::io::saved_active_layout(&scene.document).as_deref(),
+        Some("Model"),
+    );
+}
+
+#[test]
+fn active_paper_layout_survives_a_dxf_save_and_reload() {
+    let mut scene = Scene::new();
+    scene.set_current_layout("Layout1".to_string());
+
+    // Full file round-trip: write to DXF bytes, read them back.
+    let bytes = OpenCADStudio::io::save_to_bytes(&scene.document, "dxf", scene.document.version)
+        .expect("save to DXF bytes");
+    let doc = OpenCADStudio::io::load_bytes("roundtrip.dxf", bytes).expect("reload DXF bytes");
+
+    assert!(
+        !doc.header.show_model_space,
+        "$TILEMODE must persist paper space across a DXF save/reload"
+    );
+    assert_eq!(
+        OpenCADStudio::io::saved_active_layout(&doc).as_deref(),
+        Some("Layout1"),
+        "CTAB must persist the exact active tab across a DXF save/reload"
+    );
+}
+
+#[test]
+fn ctab_is_created_when_absent_then_updated_in_place() {
+    let mut scene = Scene::new();
+    let doc = &mut scene.document;
+
+    // A brand-new document carries no CTAB entry.
+    assert_eq!(OpenCADStudio::io::saved_active_layout(doc), None);
+
+    // First write creates it; a second write must update in place, not stack a
+    // duplicate entry that a reader could resolve to the stale value.
+    OpenCADStudio::io::set_saved_active_layout(doc, "Layout2");
+    assert_eq!(
+        OpenCADStudio::io::saved_active_layout(doc).as_deref(),
+        Some("Layout2")
+    );
+    OpenCADStudio::io::set_saved_active_layout(doc, "Layout3");
+    assert_eq!(
+        OpenCADStudio::io::saved_active_layout(doc).as_deref(),
+        Some("Layout3")
+    );
+}