refactor main game loop

src/app/TetrisApp.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <memory>
+
+// TetrisApp is the top-level application orchestrator.
+//
+// Responsibilities:
+// - SDL/TTF init + shutdown
+// - Asset/music loading + loading screen
+// - Main loop + state transitions
+//
+// It uses a PIMPL to keep `TetrisApp.h` light (faster builds) and to avoid leaking
+// SDL-heavy includes into every translation unit.
+class TetrisApp {
+public:
+    TetrisApp();
+    ~TetrisApp();
+
+    TetrisApp(const TetrisApp&) = delete;
+    TetrisApp& operator=(const TetrisApp&) = delete;
+
+    // Runs the application until exit is requested.
+    // Returns a non-zero exit code on initialization failure.
+    int run();
+
+private:
+    struct Impl;
+    std::unique_ptr<Impl> impl_;
+};