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/ImGuiConsole.cpp

@@ -381,9 +381,9 @@ void ImGuiConsole::render() {
     }
     ImGui::PopItemWidth();
 
-    // Keep focus on input
+    // Keep focus on input only after executing a command
     ImGui::SetItemDefaultFocus();
-    if (reclaimFocus || (visible && !ImGui::IsAnyItemActive())) {
+    if (reclaimFocus) {
         ImGui::SetKeyboardFocusHere(-1);
     }