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/SceneNode.cs

@@ -0,0 +1,50 @@
+using System.Collections.ObjectModel;
+using Ghost.Entities;
+
+namespace Ghost.Editor.Core.SceneGraph;
+
+/// <summary>
+/// Represents a Scene node in the editor hierarchy.
+/// Contains editor-only metadata like name and display state.
+/// The actual scene data (entities, components) is stored as SceneID in the runtime ECS world.
+/// </summary>
+public class SceneNode
+{
+    public string Name { get; set; }
+    public short SceneId { get; private set; }
+    public Guid SceneGuid { get; private set; }
+    
+    /// <summary>
+    /// Child entity nodes belonging to this scene.
+    /// </summary>
+    public ObservableCollection<EntityNode> Children { get; }
+
+    public SceneNode(string name, short sceneId, Guid? sceneGuid = null)
+    {
+        Name = name;
+        SceneId = sceneId;
+        SceneGuid = sceneGuid ?? Guid.NewGuid();
+        Children = new ObservableCollection<EntityNode>();
+    }
+
+    /// <summary>
+    /// Finds an entity node by its global entity ID.
+    /// Searches recursively through the hierarchy.
+    /// </summary>
+    public EntityNode? FindEntityNode(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;
+    }
+
+    public override string ToString() => $"Scene: {Name} (ID: {SceneId})";
+}