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/model/wire_model.rs

@@ -822,6 +822,11 @@ pub struct WireModel {
     /// (`display_visible` false) it is what object snaps find; drawn, it is
     /// display only and snaps skip it too ([`Self::is_display_only`]).
     pub snap_only: bool,
+    /// Plugin-owned reference-only source (API v9): the entity is display
+    /// geometry the built-in edit paths must not pick — clicks fall through
+    /// to the editable entity underneath. Rendering and snapping are
+    /// unaffected.
+    pub reference_only: bool,
     /// Whether this wire is included in plotted output.
     pub plot_visible: bool,
     /// `true` when [`fill_tris`] is a real 3-D surface (PolyfaceMesh /
@@ -910,6 +915,7 @@ impl WireModel {
             depth_override: None,
             display_visible: true,
             snap_only: false,
+            reference_only: false,
             plot_visible: true,
             fill_is_3d: false,
             fill_is_2d_solid: false,
@@ -1448,6 +1454,7 @@ impl Default for WireModel {
             depth_override: None,
             display_visible: true,
             snap_only: false,
+            reference_only: false,
             plot_visible: true,
             fill_is_3d: false,
             fill_is_2d_solid: false,