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 <noreply@anthropic.com>

crates/ocs_plugin_api/src/host.rs

@@ -142,6 +142,11 @@ pub enum TableOperation {
     BlockRename { from: String, to: String },
     /// Delete an unreferenced block definition and its contents.
     BlockDelete { name: String },
+    /// Add a new entity to a block definition, validated exactly as a new
+    /// model-space entity is. The entity's owner is set by the host. Returns the
+    /// new entity's handle. Edit and delete members with the ordinary entity
+    /// operations.
+    BlockEntityAdd { block: String, entity: EntityType },
 }
 
 /// The style tables `TableOperation` can rename, delete or make current.