feat: fixed timestep game loop + FPS counter

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

Engine/Core/engine.c

@@ -90,6 +90,12 @@ void engine_run(Engine* engine) {
         return;
     }
 
+    double fixed_dt = 1.0 / 60.0;  // 60Hz 固定时间步
+    double accumulator = 0.0;
+    double fps_timer = 0.0;
+    int fps_frames = 0;
+    int fps_display = 0;
+
     while (engine->running) {
         // 处理窗口消息
         if (!window_process_messages(engine->window)) {
@@ -100,18 +106,34 @@ void engine_run(Engine* engine) {
         // 更新计时
         timer_tick();
         engine->delta_time = timer_get_delta();
-        engine->total_time = timer_get_total();
+        engine->total_time += engine->delta_time;
         engine->frame_count++;
 
-        // 用户更新
-        if (g_update_callback) {
-            g_update_callback(engine, engine->delta_time);
+        // FPS 计算
+        fps_timer += engine->delta_time;
+        fps_frames++;
+        if (fps_timer >= 1.0) {
+            fps_display = fps_frames;
+            fps_frames = 0;
+            fps_timer -= 1.0;
         }
 
-        // 用户渲染
+        // 固定时间步逻辑更新
+        accumulator += engine->delta_time;
+        while (accumulator >= fixed_dt) {
+            if (g_update_callback) {
+                g_update_callback(engine, fixed_dt);
+            }
+            accumulator -= fixed_dt;
+        }
+
+        // 渲染
         if (g_render_callback) {
             g_render_callback(engine);
         }
+
+        // 存储 FPS 供外部读取
+        engine->fps = fps_display;
     }
 }
 

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;