feat(python): block contents (create_entity(..., block=))

BlockEntityAdd adds a validated entity to a block definition (owner set by the
host, same validation as a model-space entity, self-nesting inserts refused),
reusing the scene's block-editor routing. Deleting a block member now also
drops its handle from the block record. Real-IPC audit with an attribute
definition inside a block, DWG/DXF persistence and undo; IPC and Python unit
tests; docs.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

plugins/opencad-python/src/document_model.py

@@ -473,11 +473,17 @@ class _Document:
     def transaction(self, label):
         return _Transaction(self, label)
 
-    def create_entity(self, kind, **properties):
-        """Create one mapped entity and return its live document descriptor."""
+    def create_entity(self, kind, block=None, **properties):
+        """Create one mapped entity and return its live document descriptor.
+        With `block="Name"` the entity is added to that block definition
+        instead of the drawing (its owner is set by the host)."""
         if "kind" in properties or "handle" in properties:
             raise ValueError("kind and handle are managed by create_entity")
-        handle = ocs.add(dict(kind=kind, **_coerce_points(kind, properties)))
+        entity = dict(kind=kind, **_coerce_points(kind, properties))
+        if block is not None:
+            handle = ocs.add_to_block(str(block), entity)
+        else:
+            handle = ocs.add(entity)
         return self.entities[handle]
 
     def delete_entity(self, entity):