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/ImGuiSceneExplorer.h

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <string>
+#include <memory>
+
+class GameEngine;
+class UIDrawable;
+class UIEntity;
+class UIFrame;
+class UIGrid;
+
+/**
+ * @brief ImGui-based scene tree explorer for debugging
+ *
+ * Displays hierarchical view of all scenes and their UI elements.
+ * Allows switching between scenes and collapsing/expanding the tree.
+ * Activated by F4 key. Mutually exclusive with the console (grave key).
+ */
+class ImGuiSceneExplorer {
+public:
+    ImGuiSceneExplorer() = default;
+
+    // Core functionality
+    void render(GameEngine& engine);
+    void toggle();
+    bool isVisible() const { return visible; }
+    void setVisible(bool v) { visible = v; }
+
+    // Configuration - uses same enabled flag as console
+    static bool isEnabled();
+
+private:
+    bool visible = false;
+
+    // Tree rendering helpers
+    void renderSceneNode(GameEngine& engine, const std::string& sceneName, bool isActive);
+    void renderDrawableNode(std::shared_ptr<UIDrawable> drawable, int depth = 0);
+    void renderEntityNode(std::shared_ptr<UIEntity> entity);
+
+    // Get display name for a drawable (name or type + address)
+    std::string getDisplayName(UIDrawable* drawable);
+    std::string getEntityDisplayName(UIEntity* entity);
+
+    // Get type name string
+    const char* getTypeName(UIDrawable* drawable);
+};