Merge pull request #1747 from schoeller/reference_only_entities

Reference-only entities (plugin API v9): a scene-scoped, in-memory
designation for plugin-owned display geometry, plus HostNotification
dispatch to native plugins so they can re-mark on DocumentOpened.

Resolves the new DeltaSnapshot site in block_editor.rs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/control/sheets.rs

@@ -466,6 +466,9 @@ impl OpenCADStudio {
         self.tabs[i].current_path = None;
         self.tabs[i].is_start = false;
         self.tabs[i].dirty = true;
+        // A whole document just became resident in the tab (API v9): mint a
+        // fresh generation so the boundary announces `DocumentOpened` once.
+        self.tabs[i].note_document_loaded();
         Ok(purged)
     }