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/block_editor.rs

@@ -223,6 +223,7 @@ impl OpenCADStudio {
             active_layer,
             structure,
             parametric_constraints,
+            reference_designations: Vec::new(),
             named_parameters,
             label: format!("Edit block {}", session.block_name),
         })