feat(editor): generator undo, quit confirmation, state cleanup

- All terrain generators now undoable: crater, mesa, hill, voronoi,
  dunes, detail noise, thermal erosion, canyon, island, ridge, road,
  river, perlin noise — all wrapped with recordGeneratorUndo/commit
- Unsaved changes warning on quit: Save & Quit / Quit / Cancel dialog
- createNewTerrain clears quest editor and path capture state
- recordGeneratorUndo/commitGeneratorUndo helper methods snapshot all
  256 chunks before/after any generator operation

tools/editor/editor_app.hpp

@@ -135,6 +135,7 @@ private:
     float autoSaveTimer_ = 0.0f;
     float autoSaveInterval_ = 300.0f;
     bool autoSaveEnabled_ = true;
+    bool showQuitConfirm_ = false;
 
     // Toast notifications
     struct Toast { std::string msg; float timer; };