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_cli/lib/cli_save_game_persistence.dart

@@ -0,0 +1,55 @@
+library;
+
+import 'dart:io';
+import 'dart:typed_data';
+
+import 'package:wolf_3d_dart/wolf_3d_data_types.dart';
+import 'package:wolf_3d_dart/wolf_3d_engine.dart';
+
+/// CLI host adapter for slot-based game save persistence.
+///
+/// Files are stored under `~/.wolf3d_saves` by default and named
+/// `SAVEGAM{slot}.{ext}` where `{ext}` follows the active game version.
+class CliSaveGamePersistence implements SaveGamePersistence {
+  CliSaveGamePersistence({String? directoryPath})
+    : _directoryPath =
+          directoryPath ??
+          '${Platform.environment['HOME'] ?? '.'}/.wolf3d_saves';
+
+  final String _directoryPath;
+
+  @override
+  Future<Uint8List?> load({
+    required int slot,
+    required GameVersion version,
+  }) async {
+    try {
+      final File file = File(_slotPath(slot, version));
+      if (!file.existsSync()) {
+        return null;
+      }
+      return await file.readAsBytes();
+    } catch (_) {
+      return null;
+    }
+  }
+
+  @override
+  Future<void> save({
+    required int slot,
+    required GameVersion version,
+    required Uint8List bytes,
+  }) async {
+    final Directory dir = Directory(_directoryPath);
+    if (!dir.existsSync()) {
+      await dir.create(recursive: true);
+    }
+
+    await File(_slotPath(slot, version)).writeAsBytes(bytes, flush: true);
+  }
+
+  String _slotPath(int slot, GameVersion version) {
+    final String normalizedSlot = slot.clamp(0, 9).toString();
+    return '$_directoryPath/SAVEGAM$normalizedSlot.${version.fileExtension}';
+  }
+}