Refactor menu structure and add Flutter-specific input and persistence layers

- Moved menu-related classes to a new structure under `src/menu/`.
- Introduced `WolfMenuPresentation` to handle menu art and mappings.
- Added `MenuManager` tests to ensure menu state reflects game status.
- Implemented `FlutterRendererSettingsPersistence` and `FlutterSaveGamePersistence` for managing settings and save files on desktop platforms.
- Created `Wolf3dFlutterInput` to handle keyboard and mouse input in a Flutter environment.
- Updated README to reflect new package structure and usage instructions.

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

packages/wolf_3d_flutter/README.md

@@ -41,13 +41,25 @@ final Wolf3dFlutterEngine engine = await Wolf3dFlutterEngine(
 
 `init()` handles platform setup, audio init, and configured game-data discovery.
 
+The facade itself lives in `lib/engine/wolf3d_flutter_engine.dart` and is re-exported
+through the package barrel at `lib/wolf_3d_flutter.dart`. External consumers
+should keep importing the barrel unless they have a specific reason to target
+the engine library directly.
+
+The same pattern applies to the Flutter input adapter and the desktop
+persistence adapters: they now live in focused subdirectories and are
+re-exported through `lib/wolf_3d_flutter.dart`.
+
 For full host wiring examples, see:
 
 - `apps/wolf_3d_gui/lib/main.dart`
 
 ## Package Structure
 
-- `lib/wolf_3d_flutter.dart` — exports main host facade and public Flutter APIs.
+- `lib/wolf_3d_flutter.dart` — barrel export for the public Flutter package surface.
+- `lib/engine/wolf3d_flutter_engine.dart` — high-level engine facade and discovery bootstrap.
+- `lib/input/` — Flutter-specific input adapters.
+- `lib/persistence/` — desktop persistence adapters for saves and renderer settings.
 - `lib/renderer/` — renderer host widgets.
 - `lib/managers/` — runtime/session/display/persistence managers.
 - `lib/audio/` — platform-aware audio backends.