fix: regenerate inline sub-entity handles when copying entities

`document.add_entity` only assigns the top-level handle, leaving inline
sub-entities that carry their own — INSERT attributes and 3D-polyline
vertices — with the source's handles. Copying such an entity therefore
produced duplicate handles in the document, which corrupts the saved DWG
so other CAD apps (QCAD, DWG TrueView) refuse to open it.

Add `Scene::add_entity_clone` / `reset_clone_subhandles`, which allocate
fresh handles for the entity and every inline sub-entity, and route all
duplication paths through it: COPY/ARRAY/MIRROR-copy (via copy_entities)
and clipboard paste. (#129)

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

src/app/cmd_result.rs

@@ -557,8 +557,7 @@ impl OpenCADStudio {
                         .into_iter()
                         .map(|mut entity| {
                             crate::scene::view::dispatch::apply_transform(&mut entity, &translate);
-                            entity.common_mut().handle = acadrust::Handle::NULL;
-                            self.tabs[i].scene.add_entity(entity)
+                            self.tabs[i].scene.add_entity_clone(entity)
                         })
                         .filter(|h| !h.is_null())
                         .collect();

src/scene/mod.rs

@@ -5018,6 +5018,38 @@ impl Scene {
         self.bump_geometry_no_blocks();
     }
 
+    /// Give a freshly-cloned entity brand-new handles for every *inline*
+    /// sub-entity that stores one (INSERT attributes, 3D-polyline vertices).
+    /// `document.add_entity` only assigns the top-level handle, so without this
+    /// a copy keeps its source's sub-handles — duplicate handles that corrupt
+    /// the saved DWG (file won't reopen in other CAD apps). Vertices that don't
+    /// store a handle (LwPolyline / heavy 2D polyline) get one from the writer,
+    /// so they need no fix-up here. (#129)
+    fn reset_clone_subhandles(doc: &mut acadrust::CadDocument, entity: &mut EntityType) {
+        match entity {
+            EntityType::Insert(ins) => {
+                for att in ins.attributes.iter_mut() {
+                    att.common.handle = doc.allocate_handle();
+                }
+            }
+            EntityType::Polyline3D(p) => {
+                for v in p.vertices.iter_mut() {
+                    v.handle = doc.allocate_handle();
+                }
+            }
+            _ => {}
+        }
+    }
+
+    /// Add a freshly-cloned entity, allocating a new handle for it *and* every
+    /// inline sub-entity so the copy never shares a handle with its source.
+    /// Use this (not `add_entity`) whenever inserting a duplicate. (#129)
+    pub fn add_entity_clone(&mut self, mut entity: EntityType) -> Handle {
+        Self::reset_clone_subhandles(&mut self.document, &mut entity);
+        entity.common_mut().handle = Handle::NULL;
+        self.add_entity(entity)
+    }
+
     pub fn copy_entities(&mut self, handles: &[Handle], t: &EntityTransform) -> Vec<Handle> {
         let hatch_offset = if self.current_layout == "Model" {
             self.world_offset
@@ -5031,6 +5063,7 @@ impl Scene {
         let mut new_handles = Vec::with_capacity(clones.len());
         for mut entity in clones {
             view::dispatch::apply_transform(&mut entity, t);
+            Self::reset_clone_subhandles(&mut self.document, &mut entity);
             entity.common_mut().handle = Handle::NULL;
             let h = self.document.add_entity(entity).unwrap_or(Handle::NULL);
             if !h.is_null() {