ADD: callback function

src/runtime/i_game_callbacks.h

@@ -0,0 +1,35 @@
+#pragma once
+
+// IGameCallbacks: Interface for game logic callbacks
+// Implement this interface and pass to GameRuntime::run() to receive game loop events.
+
+namespace GameRuntime
+{
+
+class Runtime;
+
+class IGameCallbacks
+{
+public:
+    virtual ~IGameCallbacks() = default;
+
+    // Called once after runtime initialization, before the first update.
+    // Use this to load initial assets, spawn entities, set up the camera, etc.
+    virtual void on_init(Runtime& runtime) = 0;
+
+    // Called every frame with variable delta time.
+    // Use for rendering-dependent logic, input handling, camera control, etc.
+    // @param dt: Frame delta time in seconds (clamped to 0.0-0.1)
+    virtual void on_update(float dt) = 0;
+
+    // Called at fixed intervals for physics/simulation.
+    // Use for physics updates, AI tick, game state simulation, etc.
+    // @param fixed_dt: Fixed delta time in seconds (typically 1/60)
+    virtual void on_fixed_update(float fixed_dt) = 0;
+
+    // Called once before shutdown.
+    // Use for cleanup, saving state, etc.
+    virtual void on_shutdown() = 0;
+};
+
+} // namespace GameRuntime