fix(editor): separate NPC marker updates from M2 rebuild cycle

- NPC placement now only updates cheap marker geometry (no M2 reload)
- Full M2 rebuild only triggers when PLACED OBJECT count changes
  (not NPC count — NPCs use markers, not M2 instances for now)
- Split lastObjectCount_ into lastObjCount_ + lastNpcCount_ to track
  objects and NPCs independently
- Prevents the destructive clear+reload cycle that caused GPU crashes
  when rapidly placing multiple NPCs

tools/editor/editor_app.hpp

@@ -136,7 +136,8 @@ public:
     const std::vector<Toast>& getToasts() const { return toasts_; }
     void updateToasts(float dt);
 private:
-    size_t lastObjectCount_ = 0;
+    size_t lastObjCount_ = 0;
+    size_t lastNpcCount_ = 0;
     EditorMode mode_ = EditorMode::Sculpt;
     float waterHeight_ = 100.0f;
     uint16_t waterType_ = 0;