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/scene/selection.rs

@@ -204,6 +204,7 @@ impl Scene {
         let selected = handles
             .into_iter()
             .filter(|handle| self.passes_selection_filter(*handle))
+            .filter(|handle| !self.is_reference_only(*handle))
             .filter(|handle| {
                 self.document.get_entity(*handle).is_some_and(|entity| {
                     self.resident_entity_visible(
@@ -1060,7 +1061,15 @@ impl Scene {
     // ── Erase ─────────────────────────────────────────────────────────────
 
     pub fn erase_entities(&mut self, handles: &[Handle]) {
-        self.remove_entities(handles, true);
+        self.remove_entities(handles, true, true);
+    }
+
+    /// Like [`erase_entities`](Self::erase_entities) but without the
+    /// reference-only skip. The plugin owns reference-only entities and
+    /// removes them through the plugin host API; locked-layer policy stays
+    /// enforced either way.
+    pub fn erase_entities_plugin(&mut self, handles: &[Handle]) {
+        self.remove_entities(handles, true, false);
     }
 
     /// Roll back entities created by the current command after a downstream
@@ -1069,10 +1078,15 @@ impl Scene {
     /// The shared removal path keeps first-touch undo state, history objects,
     /// groups, selection, and derived caches coherent.
     pub(crate) fn rollback_new_entities(&mut self, handles: &[Handle]) {
-        self.remove_entities(handles, false);
+        self.remove_entities(handles, false, false);
     }
 
-    fn remove_entities(&mut self, handles: &[Handle], respect_layer_locks: bool) {
+    fn remove_entities(
+        &mut self,
+        handles: &[Handle],
+        respect_layer_locks: bool,
+        respect_reference_only: bool,
+    ) {
         let erase_handles = self.handles_expanded_for_leader_annotations(handles);
 
         // One pass over document.objects instead of a scan per erased handle.
@@ -1096,10 +1110,15 @@ impl Scene {
         let mut hover_changed = false;
 
         for &h in &erase_handles {
-            // Objects on a locked layer can't be erased.
+            // Objects on a locked layer can't be erased. Reference-only
+            // entities are plugin-owned: only the plugin-owned erase path
+            // may remove them.
             if respect_layer_locks && self.is_layer_locked(h) {
                 continue;
             }
+            if respect_reference_only && self.is_reference_only(h) {
+                continue;
+            }
             let settings_handle = self.document.get_entity(h).and_then(|entity| match entity {
                 EntityType::Extended(extended) => match &extended.data {
                     codec::entities::ExtendedEntityData::SectionObject(data)
@@ -1125,6 +1144,12 @@ impl Scene {
                     let before = self.document.objects.get(&manager_handle).cloned();
                     self.record_undo_object_before(manager_handle, before);
                 }
+                // The designation set is scene state, not document state: a
+                // designated entity's set entry joins the same first-touch
+                // recording so undo restores entity and designation together.
+                if self.reference_only.is_marked(h) {
+                    self.record_undo_reference_designation_before(h);
+                }
             }
             self.delete_solid_history(h);
             if let Some(settings_handle) = settings_handle {
@@ -1143,6 +1168,9 @@ impl Scene {
             }
             self.remember_removed_cache_categories(h);
             self.document.remove_entity_arc(h);
+            // The set entry dies with its entity (an absent entity is never
+            // reference-only). Undo restores it through the recorded channel.
+            self.reference_only.remove(h);
             selection_changed |= self.selected.remove(&h);
             self.selected_order.retain(|selected| *selected != h);
             if self.hover_highlight == Some(h) {