feat: Implement save game functionality with encoding/decoding

- Added SaveGameCodec for encoding and decoding save game files.
- Introduced SaveGamePersistence interface for slot-based save game persistence.
- Implemented FlutterSaveGamePersistence for file-based save management on Flutter.
- Enhanced WolfEngine to support saving and loading game states.
- Updated menu manager to include save/load game options.
- Created tests for SaveGameCodec to ensure proper functionality.

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

apps/wolf_3d_gui/lib/screens/game_screen.dart

@@ -11,6 +11,7 @@ import 'package:wolf_3d_dart/wolf_3d_engine.dart';
 import 'package:wolf_3d_dart/wolf_3d_input.dart';
 import 'package:wolf_3d_dart/wolf_3d_renderer.dart';
 import 'package:wolf_3d_flutter/renderer_settings_persistence_flutter.dart';
+import 'package:wolf_3d_flutter/save_game_persistence_flutter.dart';
 import 'package:wolf_3d_flutter/wolf_3d_flutter.dart';
 import 'package:wolf_3d_flutter/wolf_3d_input_flutter.dart';
 import 'package:wolf_3d_gui/screens/debug_tools_screen.dart';
@@ -142,6 +143,8 @@ class _GameScreenState extends State<GameScreen> {
   late final WolfEngine _engine;
   final FlutterRendererSettingsPersistence _persistence =
       FlutterRendererSettingsPersistence();
+  final FlutterSaveGamePersistence _savePersistence =
+      FlutterSaveGamePersistence();
 
   /// Mirrors [WolfRendererSettings.mode] into the Flutter renderer enum.
   RendererMode _rendererMode = RendererMode.hardware;
@@ -181,6 +184,7 @@ class _GameScreenState extends State<GameScreen> {
       onQuit: () {
         SystemNavigator.pop();
       },
+      saveGamePersistence: _savePersistence,
     );
     _syncRendererModeFrom(_engine.rendererSettings);
     _loadPersistedSettings();