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/terrain_editor.hpp

@@ -69,6 +69,7 @@ private:
     void applyRaise(float dt);
     void applySmooth(float dt);
     void applyFlatten(float dt);
+    void applyErode(float dt);
     void stitchEdges(int chunkIdx);
 
     std::vector<int> getAffectedChunks(const glm::vec3& center, float radius) const;