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/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,
 }