feat(properties): label xref Insert as "External Reference"

Match AutoCAD properties panel — Insert whose block_record carries
is_xref/is_xref_overlay now shows "External Reference" instead of
"Block Reference". Silence dead_code warning on XrefStatus::Unloaded
(intentional, awaiting on-disk Unload-bit detection).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/properties.rs

@@ -174,7 +174,24 @@ impl H7CAD {
                             });
                         }
                     }
-                    let title = entity_type_label(entity);
+                    let title = match entity {
+                        acadrust::EntityType::Insert(ins) => {
+                            let is_xref = self.tabs[i]
+                                .scene
+                                .document
+                                .block_records
+                                .iter()
+                                .find(|br| br.name == ins.block_name)
+                                .map(|br| br.flags.is_xref || br.flags.is_xref_overlay)
+                                .unwrap_or(false);
+                            if is_xref {
+                                "External Reference".to_string()
+                            } else {
+                                entity_type_label(entity)
+                            }
+                        }
+                        _ => entity_type_label(entity),
+                    };
                     ui::PropertiesPanel {
                         choice_combos: sections
                             .iter()

src/io/xref.rs

@@ -17,6 +17,7 @@ pub enum XrefStatus {
     NotFound,
     /// XRef is marked Unloaded in the host DWG — we honor that and
     /// skip resolving the external file. The user can re-load via UI.
+    #[allow(dead_code)]
     Unloaded,
 }