feat(editor): erosion brush, NPC load, auto-save

- Erode brush mode: simulates water erosion by moving height downhill
  based on slope, creating natural drainage patterns and gullies
- NPC JSON loader: File > Load NPCs parses saved creatures.json back
  into the spawn list (round-trip save/load now works)
- Auto-save: every 5 minutes when unsaved changes exist, exports the
  full zone (ADT + WDT + creatures) to the output directory
- Sculpt mode now has 6 brush types: Raise/Lower/Smooth/Flatten/Level/Erode

tools/editor/editor_app.hpp

@@ -116,6 +116,9 @@ private:
     bool openContextMenu_ = false;
     std::string lastSavePath_;
     std::vector<CameraBookmark> bookmarks_;
+    float autoSaveTimer_ = 0.0f;
+    float autoSaveInterval_ = 300.0f; // 5 minutes
+    bool autoSaveEnabled_ = true;
     size_t lastObjectCount_ = 0;
     EditorMode mode_ = EditorMode::Sculpt;
     float waterHeight_ = 100.0f;