feat: add play / editor mode

tests/serialization_tests.cpp

@@ -334,6 +334,40 @@ namespace {
 
         SceneManager::GetInstance().Destroy();
     }
+
+    void testPlayModeSnapshotRestore()
+    {
+        Scene& scene = SceneManager::GetInstance().CreateScene("snapshot");
+        GameObject* box = scene.CreateGameObject("box");
+        box->GetTransform().SetLocalPosition({1.f, 2.f, 3.f});
+        scene.CreateGameObject("other");
+        scene.CommitPendingAdditions();
+        scene.SetSelectedObject(box->GetId());
+
+        auto snapshot = SceneSerializer::Capture(scene);
+        check(snapshot.has_value(), "a scene must be capturable without a file");
+        (*snapshot)["selected"] = box->GetId();
+
+        // What play mode does to the scene before Stop restores it
+        box->GetTransform().SetLocalPosition({9.f, 9.f, 9.f});
+        scene.CreateGameObject("spawned during play");
+        scene.CommitPendingAdditions();
+
+        check(SceneSerializer::LoadFromJson(scene, *snapshot), "restoring a snapshot must succeed");
+        scene.CommitPendingAdditions();
+        check(scene.GetObjects().size() == 2 && findByName(scene, "spawned during play") == nullptr,
+              "restoring must drop objects created after the snapshot");
+        const GameObject* restored = findByName(scene, "box");
+        check(restored != nullptr && close(restored->GetTransform().GetLocalPosition().x, 1.f),
+              "restoring must bring back the captured transforms");
+        check(scene.GetSelectedObject() == restored, "the selection must follow the restored object");
+
+        check(!SceneSerializer::LoadFromJson(scene, nlohmann::json{{"objects", 5}}),
+              "an invalid snapshot must be rejected");
+        check(scene.GetObjects().size() == 2, "a rejected snapshot must leave the scene alone");
+
+        SceneManager::GetInstance().Destroy();
+    }
 }
 
 void RunSerializationTests()
@@ -350,4 +384,5 @@ void RunSerializationTests()
     testSceneFileIdsAreLocal();
     testMaterialJsonRoundTrip();
     testSceneValidatesEmbeddedMaterials();
+    testPlayModeSnapshotRestore();
 }