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/terrain_editor.hpp

@@ -163,6 +163,9 @@ private:
     std::vector<StampVertex> stampData_;
     glm::vec3 stampCenter_{0};
 
+    void recordGeneratorUndo();
+    void commitGeneratorUndo();
+
     pipeline::ADTTerrain* terrain_ = nullptr;
     EditorBrush brush_;
     EditorHistory history_;