feat: Refactor MD5 hashing and update save game codec for compatibility with new payload format

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

packages/wolf_3d_dart/test/engine/save_game_codec_test.dart

@@ -183,9 +183,10 @@ void main() {
     final Uint8List encoded = codec.encode(file);
     final SaveGameFile decoded = codec.decode(encoded);
 
-    expect(decoded.slot, 0);
+    expect(decoded.slot, 2);
     expect(decoded.description, 'Compatible Block Save');
-    expect(decoded.createdAtMs, 0);
+    expect(decoded.createdAtMs, 1234);
+    expect(decoded.dataVersionName, file.dataVersionName);
     expect(
       decoded.snapshot.currentEpisodeIndex,
       file.snapshot.currentEpisodeIndex,
@@ -193,6 +194,45 @@ void main() {
     expect(decoded.snapshot.currentLevelIndex, file.snapshot.currentLevelIndex);
   });
 
+  test('CompatibleSaveGameCodec preserves entity state fidelity', () {
+    final WolfEngine engine = _buildEngine();
+    engine.init();
+
+    final Guard guard =
+        EntityRegistry.spawn(
+                MapObject.guardStart,
+                8.5,
+                7.5,
+                Difficulty.medium,
+                engine.data.sprites.length,
+                registry: engine.data.registry,
+              )!
+              as Guard
+          ..health = 5
+          ..state = EntityState.dead;
+    engine.entities = <Entity>[guard];
+
+    final CompatibleSaveGameCodec codec = CompatibleSaveGameCodec();
+    final SaveGameFile decoded = codec.decode(
+      codec.encode(
+        SaveGameFile(
+          slot: 0,
+          gameVersion: engine.data.version,
+          dataVersionName: engine.data.dataVersion.name,
+          description: 'Entity Fidelity',
+          createdAtMs: 1,
+          snapshot: engine.captureSaveState(),
+          checksum: 0,
+        ),
+      ),
+    );
+
+    expect(decoded.snapshot.entities, hasLength(1));
+    expect(decoded.snapshot.entities.first.kind, 'Guard');
+    expect(decoded.snapshot.entities.first.state, EntityState.dead);
+    expect(decoded.snapshot.entities.first.extraData['health'], 5);
+  });
+
   test('CompatibleSaveGameCodec decodes old envelope payload format', () {
     final WolfEngine engine = _buildEngine();
     engine.init();