feat(editor): NPC scatter tool, adjacent tile creation, multi-tile prep

- Scatter tool: place N creatures in a radius around cursor position
  with random rotation and uniform disk distribution
- File > Add Adjacent Tile: creates and exports a blank tile N/S/E/W
  of current (foundation for multi-tile zone editing)
- Scatter UI: count slider (1-30), radius slider (10-200)
- Scatter places all copies with same stats/behavior as template

tools/editor/editor_app.hpp

@@ -64,6 +64,9 @@ public:
     void startGizmoMode(TransformMode mode);
     void setGizmoAxis(TransformAxis axis);
     void snapSelectedToGround();
+
+    // Multi-tile support
+    void addAdjacentTile(int offsetX, int offsetY);
     TransformGizmo& getGizmo() { return viewport_.getGizmo(); }
     bool shouldOpenContextMenu() const { return openContextMenu_; }
     void clearContextMenuFlag() { openContextMenu_ = false; }