feat: Add CompatibleSaveGameCodec for legacy W3DS support and enhance SaveGamePersistence with existence check

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

packages/wolf_3d_dart/lib/src/engine/save/save_game_codec.dart

@@ -578,6 +578,21 @@ class OriginalLayoutEnvelopeSaveGameCodec extends SaveGameCodec {
   }
 }
 
+/// Writes classic-envelope saves while remaining able to read legacy W3DS
+/// saves created by earlier engine builds.
+class CompatibleSaveGameCodec extends OriginalLayoutEnvelopeSaveGameCodec {
+  final SaveGameCodec _legacyCodec = SaveGameCodec();
+
+  @override
+  SaveGameFile decode(Uint8List bytes) {
+    try {
+      return super.decode(bytes);
+    } on FormatException {
+      return _legacyCodec.decode(bytes);
+    }
+  }
+}
+
 class _BinaryWriter {
   final BytesBuilder _builder = BytesBuilder(copy: false);
 

packages/wolf_3d_dart/lib/src/engine/save/save_game_persistence.dart

@@ -9,6 +9,12 @@ abstract class SaveGamePersistence {
   /// Loads raw bytes for [slot] and [version], or `null` when no save exists.
   Future<Uint8List?> load({required int slot, required GameVersion version});
 
+  /// Returns whether a non-empty save exists for [slot] and [version].
+  Future<bool> exists({required int slot, required GameVersion version}) async {
+    final Uint8List? bytes = await load(slot: slot, version: version);
+    return bytes != null && bytes.isNotEmpty;
+  }
+
   /// Persists [bytes] for [slot] and [version].
   Future<void> save({
     required int slot,

packages/wolf_3d_dart/lib/src/engine/wolf_3d_engine_base.dart

@@ -50,7 +50,7 @@ class WolfEngine {
          'Provide either data or a non-empty availableGames list.',
        ),
        _availableGames = availableGames ?? <WolfensteinData>[data!],
-       saveGameCodec = saveGameCodec ?? SaveGameCodec(),
+       saveGameCodec = saveGameCodec ?? CompatibleSaveGameCodec(),
        audio = engineAudio ?? CliSilentAudio(),
        doorManager = DoorManager(
          onPlaySound: (effect) => engineAudio?.playSoundEffect(effect),
@@ -498,18 +498,10 @@ class WolfEngine {
       return;
     }
 
-    bool hasSave = false;
-    for (final game in _availableGames) {
-      final Uint8List? bytes = await saveGamePersistence!.load(
-        slot: defaultSaveSlot,
-        version: game.version,
-      );
-      if (bytes != null && bytes.isNotEmpty) {
-        hasSave = true;
-        break;
-      }
-    }
-
+    final bool hasSave = await saveGamePersistence!.exists(
+      slot: defaultSaveSlot,
+      version: data.version,
+    );
     _setLoadGameAvailability(hasSave);
   }
 
@@ -921,6 +913,9 @@ class WolfEngine {
       _currentEpisodeIndex = 0;
       onEpisodeSelected?.call(null);
       menuManager.clearEpisodeSelection();
+      if (saveGamePersistence != null) {
+        unawaited(_refreshLoadGameAvailability());
+      }
       menuManager.beginIntroSplash(
         includeRetailWarning: data.version == GameVersion.retail,
       );