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>

packages/wolf_3d_flutter/lib/save_game_persistence_flutter.dart

@@ -0,0 +1,78 @@
+library;
+
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
+import 'package:wolf_3d_dart/wolf_3d_data_types.dart';
+import 'package:wolf_3d_dart/wolf_3d_engine.dart';
+
+/// Flutter desktop adapter for slot-based game save persistence.
+class FlutterSaveGamePersistence implements SaveGamePersistence {
+  FlutterSaveGamePersistence({String? directoryPath})
+    : _directoryPath = directoryPath;
+
+  final String? _directoryPath;
+  String? _resolvedDirectoryPath;
+
+  Future<String> _resolveDirectoryPath() async {
+    if (_resolvedDirectoryPath != null) {
+      return _resolvedDirectoryPath!;
+    }
+
+    if (_directoryPath != null) {
+      _resolvedDirectoryPath = _directoryPath;
+      return _resolvedDirectoryPath!;
+    }
+
+    final String home =
+        Platform.environment['HOME'] ?? Platform.environment['APPDATA'] ?? '.';
+    _resolvedDirectoryPath = '$home/.wolf3d_saves';
+    return _resolvedDirectoryPath!;
+  }
+
+  @override
+  Future<Uint8List?> load({
+    required int slot,
+    required GameVersion version,
+  }) async {
+    if (kIsWeb) {
+      return null;
+    }
+
+    try {
+      final String dirPath = await _resolveDirectoryPath();
+      final File file = File(_slotPath(dirPath, slot, version));
+      if (!file.existsSync()) {
+        return null;
+      }
+      return await file.readAsBytes();
+    } catch (_) {
+      return null;
+    }
+  }
+
+  @override
+  Future<void> save({
+    required int slot,
+    required GameVersion version,
+    required Uint8List bytes,
+  }) async {
+    if (kIsWeb) {
+      return;
+    }
+
+    final String dirPath = await _resolveDirectoryPath();
+    final Directory dir = Directory(dirPath);
+    if (!dir.existsSync()) {
+      await dir.create(recursive: true);
+    }
+    await File(
+      _slotPath(dirPath, slot, version),
+    ).writeAsBytes(bytes, flush: true);
+  }
+
+  String _slotPath(String dirPath, int slot, GameVersion version) {
+    final String normalizedSlot = slot.clamp(0, 9).toString();
+    return '$dirPath/SAVEGAM$normalizedSlot.${version.fileExtension}';
+  }
+}

packages/wolf_3d_flutter/lib/wolf_3d_flutter.dart

@@ -85,6 +85,7 @@ class Wolf3d {
   WolfEngine launchEngine({
     required void Function() onGameWon,
     void Function()? onQuit,
+    SaveGamePersistence? saveGamePersistence,
     WolfRendererCapabilities? rendererCapabilities,
     WolfRendererSettings? rendererSettings,
     void Function(WolfRendererSettings settings)? onRendererSettingsChanged,
@@ -109,6 +110,7 @@ class Wolf3d {
       // so backing out of the top-level menu should not pop the route.
       onMenuExit: () {},
       onQuit: onQuit,
+      saveGamePersistence: saveGamePersistence,
       rendererCapabilities: rendererCapabilities,
       rendererSettings: rendererSettings,
       onRendererSettingsChanged: onRendererSettingsChanged,