fix(reorder): trap layer-list drag drop as a data change and persist it

List_PreviewMouseMove reorders StagedScene.Elements via RemoveAt/Insert,
bypassing SceneGraph's mutation surface, but never scheduled a save — the
new z-order was lost on restart. The drop now sets _dragReordered and
EndListDrag funnels it through MainViewModel.OnSceneElementsReordered()
(InvalidateBake + ScheduleSave, same background-save path as every other
mutation). Integration test (RealApp + temp DB) reproduces the exact
code-behind mutation and asserts the debounced save lands the new Source
SortOrder. Derivation reference: standard WPF ItemsControl drag-reorder
pattern (OBS layering semantics: bottom-most layer = index 0).

Controls/LeftPanel.xaml.cs

@@ -20,6 +20,7 @@ public partial class LeftPanel : UserControl
     private int _dragIndex = -1;
     private bool _isDragging;
     private object? _lastHoveredItem;
+    private bool _dragReordered;
 
     public LeftPanel()
     {
@@ -159,6 +160,7 @@ public partial class LeftPanel : UserControl
         _dragStartPoint = e.GetPosition(listBox);
         _isDragging = false;
         _lastHoveredItem = null;
+        _dragReordered = false;
     }
 
     private void List_PreviewMouseMove(object sender, MouseEventArgs e)
@@ -196,6 +198,7 @@ public partial class LeftPanel : UserControl
         items.Insert(targetIndex, dragged);
         _dragIndex = targetIndex;
         _lastHoveredItem = targetItem;
+        _dragReordered = true;
     }
 
     private void List_PreviewMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
@@ -207,9 +210,13 @@ public partial class LeftPanel : UserControl
     {
         if (_isDragging)
             listBox.ReleaseMouseCapture();
+        var reordered = _dragReordered;
         _dragIndex = -1;
         _isDragging = false;
         _lastHoveredItem = null;
+        _dragReordered = false;
+        if (reordered && DataContext is MainViewModel vm)
+            vm.OnSceneElementsReordered();
     }
 
     private static ListBoxItem? FindItemContainerAt(ListBox listBox, Point position)