feat: implement complete scene graph system with hierarchical editor support

- Add SceneNode and EntityNode classes for editor-only metadata storage
- Implement SceneGraph view-model with O(1) entity lookup via internal caching
- Create IdRemapTable for file-local to global entity ID remapping on load
- Implement SceneSerializationContext for load/save operation tracking
- Add JSON-serializable SceneAssetData, EntityData, and ComponentData models
- Implement SceneSerializer for save/load with validation and reference remapping
- Add comprehensive documentation: README.md, IMPLEMENTATION_GUIDE.md, SYSTEM_SUMMARY.md
- Update Ghost.Editor.Core.csproj to reference Ghost.Entities assembly
- Support parent-child relationships via Hierarchy component
- Enforce no cross-scene entity references
- Keep runtime minimal: only SceneID, Hierarchy, LocalToWorld components
- All editor metadata (names, UI state) stored in editor-only SceneNode/EntityNode classes

This implements the architecture from SceneGraph Plan.md with clean separation of concerns,
minimal runtime footprint, and AOT compatibility.

Ghost.Editor.Core/SceneGraph/EntityNode.cs

@@ -0,0 +1,105 @@
+using Ghost.Entities;
+using System.Collections.ObjectModel;
+
+namespace Ghost.Editor.Core.SceneGraph;
+
+/// <summary>
+/// Represents an Entity node in the editor hierarchy.
+/// Contains editor-only metadata like name and selection state.
+/// References the actual entity data in the ECS world via EntityId.
+/// </summary>
+public class EntityNode
+{
+    public string Name { get; set; }
+    public Entity EntityId { get; private set; }
+    
+    /// <summary>
+    /// File-local ID within the scene (used for serialization).
+    /// Only set when loaded from a scene file; may be -1 if not yet assigned.
+    /// </summary>
+    public int FileLocalId { get; set; } = -1;
+
+    /// <summary>
+    /// Child entity nodes (parent-child relationships in hierarchy).
+    /// </summary>
+    public ObservableCollection<EntityNode> Children { get; }
+
+    /// <summary>
+    /// Reference to parent entity node, if any.
+    /// </summary>
+    public EntityNode? ParentNode { get; set; }
+
+    /// <summary>
+    /// Whether this node is expanded in the editor UI.
+    /// </summary>
+    public bool IsExpanded { get; set; }
+
+    /// <summary>
+    /// Whether this node is selected in the editor UI.
+    /// </summary>
+    public bool IsSelected { get; set; }
+
+    public EntityNode(string name, Entity entityId)
+    {
+        Name = name;
+        EntityId = entityId;
+        Children = new ObservableCollection<EntityNode>();
+        IsExpanded = false;
+        IsSelected = false;
+    }
+
+    /// <summary>
+    /// Finds a child entity node recursively by its global entity ID.
+    /// </summary>
+    public EntityNode? FindRecursive(Entity entityId)
+    {
+        foreach (var child in Children)
+        {
+            if (child.EntityId == entityId)
+                return child;
+
+            var found = child.FindRecursive(entityId);
+            if (found != null)
+                return found;
+        }
+
+        return null;
+    }
+
+    /// <summary>
+    /// Gets the depth of this node in the hierarchy.
+    /// Root nodes have depth 0.
+    /// </summary>
+    public int GetDepth()
+    {
+        int depth = 0;
+        var current = ParentNode;
+        while (current != null)
+        {
+            depth++;
+            current = current.ParentNode;
+        }
+        return depth;
+    }
+
+    /// <summary>
+    /// Gets all descendant nodes in breadth-first order.
+    /// </summary>
+    public IEnumerable<EntityNode> GetAllDescendants()
+    {
+        var queue = new Queue<EntityNode>();
+        queue.Enqueue(this);
+
+        while (queue.Count > 0)
+        {
+            var node = queue.Dequeue();
+            foreach (var child in node.Children)
+            {
+                yield return child;
+                queue.Enqueue(child);
+            }
+        }
+    }
+
+    public override string ToString() => $"Entity: {Name} (ID: {EntityId})";
+}