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.cpp

@@ -81,6 +81,16 @@ void EditorApp::run() {
         // Handle pending UI actions
         ui_.processActions(*this);
 
+        // Auto-save
+        if (autoSaveEnabled_ && terrain_.isLoaded() && terrainEditor_.hasUnsavedChanges()) {
+            autoSaveTimer_ += dt;
+            if (autoSaveTimer_ >= autoSaveInterval_) {
+                autoSaveTimer_ = 0.0f;
+                quickSave();
+                LOG_INFO("Auto-saved zone");
+            }
+        }
+
         // Refresh dirty terrain chunks
         refreshDirtyChunks();