docs(genui): document restricted primitives exports breaking change in CHANGELOG.md

Author: gspencergoog

packages/genui/CHANGELOG.md

@@ -4,6 +4,7 @@
 
 - **BREAKING**: Changed `PromptBuilder.chat` and `PromptBuilder.custom` from synchronous factory constructors to asynchronous static methods (`createChat` and `createCustom`) to support asynchronous asset loading.
 - **BREAKING**: Changed `_loadSchemas` return type to a named record structure.
+- **BREAKING**: Restricted public API surface of low-level `primitives` exports. Only `CancellationException`, `CancellationSignal`, `JsonMap`, `basicCatalogId`, `configureLogging`, `genUiLogger`, and `generateId` are now exported from `package:genui/genui.dart`.
 - **Refactor**: Extracted exception mapping logic to a private helper `_errorToMap` in `SurfaceController`.
 - **Refactor**: Centralized and shared common schema registry initialization helper.
 - **Refactor**: Extracted mock binary messenger asset setup to a shared helper for test reuse.