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_brush.hpp

@@ -10,7 +10,8 @@ enum class BrushMode {
     Lower,
     Smooth,
     Flatten,
-    Level
+    Level,
+    Erode
 };
 
 struct BrushSettings {