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/ipc/v4/protocol.rs

@@ -141,6 +141,10 @@ mod tests {
             TableOperation::BlockModify { name: "B".into(), description: None, explodable: Some(false), scale_uniformly: Some(true) },
             TableOperation::BlockRename { from: "B".into(), to: "C".into() },
             TableOperation::BlockDelete { name: "C".into() },
+            TableOperation::BlockEntityAdd {
+                block: "C".into(),
+                entity: acadrust::EntityType::Line(acadrust::entities::Line::new()),
+            },
         ] {
             let bytes = bincode::serialize(&PluginRequest::TableOperation { operation: operation.clone() }).unwrap();
             assert!(matches!(bincode::deserialize::<PluginRequest>(&bytes).unwrap(),