feat(Phase 5): Complete Window/Scene Architecture

- Window singleton with properties (resolution, fullscreen, vsync, title)
- OOP Scene support with lifecycle methods (on_enter, on_exit, on_keypress, update)
- Window resize events trigger scene.on_resize callbacks
- Scene transitions (fade, slide_left/right/up/down) with smooth animations
- Full integration of Python Scene objects with C++ engine

All Phase 5 tasks (#34, #1, #61, #105) completed successfully.

src/SceneTransition.h

@@ -0,0 +1,42 @@
+#pragma once
+#include "Common.h"
+#include <SFML/Graphics.hpp>
+#include <string>
+#include <memory>
+
+enum class TransitionType {
+    None,
+    Fade,
+    SlideLeft,
+    SlideRight,
+    SlideUp,
+    SlideDown
+};
+
+class SceneTransition {
+public:
+    TransitionType type = TransitionType::None;
+    float duration = 0.0f;
+    float elapsed = 0.0f;
+    std::string fromScene;
+    std::string toScene;
+    
+    // Render textures for transition
+    std::unique_ptr<sf::RenderTexture> oldSceneTexture;
+    std::unique_ptr<sf::RenderTexture> newSceneTexture;
+    
+    // Sprites for rendering textures
+    sf::Sprite oldSprite;
+    sf::Sprite newSprite;
+    
+    SceneTransition() = default;
+    
+    void start(TransitionType t, const std::string& from, const std::string& to, float dur);
+    void update(float dt);
+    void render(sf::RenderTarget& target);
+    bool isComplete() const { return elapsed >= duration; }
+    float getProgress() const { return duration > 0 ? std::min(elapsed / duration, 1.0f) : 1.0f; }
+    
+    // Easing function for smooth transitions
+    static float easeInOut(float t);
+};