restore c++ backend using swift 6's improved interop

Sources/CppBackend/ball.cpp

@@ -30,6 +30,8 @@ void Ball::update(float deltaTime) noexcept {
 }
 
 
+BallWorld::BallWorld() noexcept : balls() {}
+
 void BallWorld::add(Ball::vec2f pos, float angle, float ballSize) noexcept {
   balls.emplace_back(Ball{ pos, angle, ballSize });
 }

Sources/CppBackend/ball.hpp

@@ -34,6 +34,9 @@ public:
 struct BallWorld {
   std::vector<Ball> balls;
 
+  BallWorld() noexcept;
+  virtual ~BallWorld() noexcept = default;
+
   void add(Ball::vec2f pos, float angle, float ballSize) noexcept;
   void update(float deltaTime) noexcept;
 };