feat: implement scene graph system with ECS hierarchy support

Add hierarchical scene graph for editor with TreeView UI, runtime
HierarchyUtility for parent/child linked-list management, and
incremental sync between editor world and scene graph nodes.

- SceneGraphNode/SceneNode/EntityNode with World, Scene, Entity refs
- SceneGraphBuilder — construct tree from ECS World queries
- HierarchyUtility — SetParent, RemoveParent, IsAncestor, cascade destroy
- EditorWorldService + SceneGraphSyncService — editor world lifecycle & incremental sync
- Hierarchy.xaml — TreeView with DataTemplate + SceneGraphTemplateSelector
- 25 unit tests covering hierarchy ops and scene graph building

src/Runtime/Ghost.Entities/System.cs

@@ -332,6 +332,11 @@ public abstract class SystemGroup : ISystem
 
     public void Initialize(ref readonly SystemAPI systemAPI)
     {
+        if (_systems.Count == 0)
+        {
+            return;
+        }
+
         ThrowIfNotSorted();
 
         foreach (var system in _sortedSystems!)
@@ -342,6 +347,11 @@ public abstract class SystemGroup : ISystem
 
     public void Update(ref readonly SystemAPI systemAPI)
     {
+        if (_systems.Count == 0)
+        {
+            return;
+        }
+
         ThrowIfNotSorted();
 
         foreach (var system in _sortedSystems!)
@@ -352,6 +362,11 @@ public abstract class SystemGroup : ISystem
 
     public void Cleanup(ref readonly SystemAPI systemAPI)
     {
+        if (_systems.Count == 0)
+        {
+            return;
+        }
+
         ThrowIfNotSorted();
 
         foreach (var system in _sortedSystems!)
@@ -399,6 +414,11 @@ public sealed class SystemManager : IDisposable
 
     internal void InitializeAll(TimeData timeData)
     {
+        if (_systems.Count == 0)
+        {
+            return;
+        }
+
         var systemAPI = new SystemAPI
         {
             Time = timeData,
@@ -413,6 +433,11 @@ public sealed class SystemManager : IDisposable
 
     internal void UpdateAll(TimeData timeData)
     {
+        if (_systems.Count == 0)
+        {
+            return;
+        }
+
         var systemAPI = new SystemAPI
         {
             Time = timeData,
@@ -427,6 +452,11 @@ public sealed class SystemManager : IDisposable
 
     internal void CleanupAll(TimeData timeData)
     {
+        if (_systems.Count == 0)
+        {
+            return;
+        }
+
         var systemAPI = new SystemAPI
         {
             Time = timeData,