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/v4/client.rs

@@ -730,6 +730,34 @@ impl HostApi for V4PluginHostApi {
             }
         }
     }
+
+    fn add_layer(&mut self, config: crate::host::LayerConfig) -> Option<Handle> {
+        match self.request(PluginRequest::AddLayer(config)) {
+            Ok(PluginResponse::OptHandle(h)) => h,
+            Ok(other) => {
+                eprintln!("[plugin] unexpected AddLayer response: {other:?}");
+                None
+            }
+            Err(e) => {
+                eprintln!("[plugin] AddLayer request failed: {e}");
+                None
+            }
+        }
+    }
+
+    fn modify_layer(&mut self, config: crate::host::LayerConfig) -> bool {
+        match self.request(PluginRequest::ModifyLayer(config)) {
+            Ok(PluginResponse::Bool(b)) => b,
+            Ok(other) => {
+                eprintln!("[plugin] unexpected ModifyLayer response: {other:?}");
+                false
+            }
+            Err(e) => {
+                eprintln!("[plugin] ModifyLayer request failed: {e}");
+                false
+            }
+        }
+    }
 }
 
 #[cfg(test)]