Add ImGui Scene Explorer (F4) for runtime object inspection (#136)

New Features:
- Scene Explorer window (F4) displays hierarchical tree of all scenes
- Shows UIDrawable hierarchy with type, name, and visibility status
- Click scene name to switch active scene
- Double-click drawables to toggle visibility
- Displays Python repr() for cached objects, enabling custom class debugging
- Entity display within Grid nodes

Bug Fixes:
- Fix PythonObjectCache re-registration: when retrieving objects from
  collections, newly created Python wrappers are now re-registered in
  the cache. Previously, inline-created objects (e.g.,
  scene.children.append(Frame(...))) would lose their cache entry when
  the temporary Python object was GC'd, causing repeated wrapper
  allocation on each access.
- Fix console focus stealing: removed aggressive focus reclaim that
  caused title bar flashing when clicking in Scene Explorer

Infrastructure:
- Add GameEngine::getSceneNames() to expose scene list for explorer
- Scene Explorer uses same enabled flag as console (ImGuiConsole::isEnabled())

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/GameEngine.h

@@ -11,6 +11,7 @@
 #include "SceneTransition.h"
 #include "Profiler.h"
 #include "ImGuiConsole.h"
+#include "ImGuiSceneExplorer.h"
 #include <memory>
 #include <sstream>
 #include <mutex>
@@ -191,6 +192,7 @@ private:
 
     // ImGui console overlay
     ImGuiConsole console;
+    ImGuiSceneExplorer sceneExplorer;
     bool imguiInitialized = false;
 
     // #219 - Thread synchronization for background Python threads
@@ -214,6 +216,7 @@ public:
     ~GameEngine();
     Scene* currentScene();
     Scene* getScene(const std::string& name);  // #118: Get scene by name
+    std::vector<std::string> getSceneNames() const;  // #136: Get all scene names for explorer
     void changeScene(std::string);
     void changeScene(std::string sceneName, TransitionType transitionType, float duration);
     void createScene(std::string);