feat(editor): smooth entire tile, snap-to-ground toggle, object list improvements

- Smooth Entire Tile: global smoothing pass with configurable iterations
  (1-10). Smooths across chunk boundaries for seamless results. Updates
  inner vertices from smoothed outer grid. Great after noise generation.
- Snap to Ground checkbox: on by default, objects placed at terrain
  surface. Disable for floating/airborne objects.
- Random Rotation + Snap Ground checkboxes side-by-side for fast setup
- Toast notifications on noise apply and smooth operations
- Smooth pass uses cross-chunk neighbor averaging for edge continuity

tools/editor/object_placer.hpp

@@ -58,6 +58,8 @@ public:
     void setPlacementScale(float s) { placementScale_ = s; }
     bool getRandomRotation() const { return randomRotation_; }
     void setRandomRotation(bool v) { randomRotation_ = v; }
+    bool getSnapToGround() const { return snapToGround_; }
+    void setSnapToGround(bool v) { snapToGround_ = v; }
 
     // Undo last placement
     bool canUndoPlace() const { return !undoStack_.empty(); }
@@ -81,6 +83,7 @@ private:
     float placementRotY_ = 0.0f;
     float placementScale_ = 1.0f;
     bool randomRotation_ = false;
+    bool snapToGround_ = true;
 };
 
 } // namespace editor