feat(plugin): add layer creation and modification host API

- Add LayerConfig struct and add_layer / modify_layer methods to HostApi trait
- Implement IPC request/response serialization for AddLayer and ModifyLayer
- Implement HostSession methods with CAD property defaults and selective modification
- Refresh layer panel UI and invalidate scene layer dependencies upon layer changes
- Add unit tests for add_layer with defaults and partial modify_layer

crates/ocs_plugin_api/src/ipc/server.rs

@@ -77,5 +77,7 @@ pub fn handle_plugin_request(
         DocumentPath { tab_id } => PluginResponse::DocumentPath(
             host.document_path(tab_id).map(|path| path.into_os_string()),
         ),
+        AddLayer(config) => PluginResponse::OptHandle(host.add_layer(config)),
+        ModifyLayer(config) => PluginResponse::Bool(host.modify_layer(config)),
     }
 }