feat: fixed timestep game loop + FPS counter

- 60Hz fixed timestep with accumulator
- FPS display via engine->fps
- Proper delta_time accumulation

Engine/Core/engine.h

@@ -27,6 +27,7 @@ typedef struct {
     void* renderer;
     double delta_time;
     double total_time;
+    int fps;
     uint64_t frame_count;
 } Engine;