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/Serialization/SceneAssetData.cs

@@ -0,0 +1,98 @@
+using System.Text.Json.Serialization;
+
+namespace Ghost.Editor.Core.SceneGraph.Serialization;
+
+/// <summary>
+/// JSON-serializable representation of a component instance.
+/// Only used in the editor for saving/loading scenes.
+/// </summary>
+[Serializable]
+public class ComponentData
+{
+    /// <summary>
+    /// Fully qualified type name of the component (e.g., "Ghost.Engine.Components.Transform").
+    /// </summary>
+    [JsonPropertyName("type")]
+    public string ComponentTypeName { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Serialized component data as a dictionary.
+    /// Field names map to JSON values.
+    /// </summary>
+    [JsonPropertyName("data")]
+    public Dictionary<string, object?> Data { get; set; } = new();
+}
+
+/// <summary>
+/// JSON-serializable representation of an entity within a scene.
+/// Only used in the editor for saving/loading scenes.
+/// 
+/// The index in the entities list corresponds to the file-local ID.
+/// </summary>
+[Serializable]
+public class EntityData
+{
+    /// <summary>
+    /// File-local entity ID within the scene.
+    /// Set by the serializer based on position in the entities list.
+    /// </summary>
+    [JsonPropertyName("fileLocalId")]
+    public int FileLocalId { get; set; }
+
+    /// <summary>
+    /// Editor-only name for the entity.
+    /// </summary>
+    [JsonPropertyName("name")]
+    public string Name { get; set; } = "Entity";
+
+    /// <summary>
+    /// File-local ID of the parent entity, or -1 if root.
+    /// </summary>
+    [JsonPropertyName("parentFileLocalId")]
+    public int ParentFileLocalId { get; set; } = -1;
+
+    /// <summary>
+    /// All components attached to this entity.
+    /// </summary>
+    [JsonPropertyName("components")]
+    public List<ComponentData> Components { get; set; } = new();
+}
+
+/// <summary>
+/// JSON-serializable representation of a scene.
+/// Only used in the editor for saving/loading scenes.
+/// </summary>
+[Serializable]
+public class SceneAssetData
+{
+    /// <summary>
+    /// Scene metadata version for forward compatibility.
+    /// </summary>
+    [JsonPropertyName("version")]
+    public int Version { get; set; } = 1;
+
+    /// <summary>
+    /// Unique identifier for this scene (GUID).
+    /// </summary>
+    [JsonPropertyName("sceneGuid")]
+    public Guid SceneGuid { get; set; } = Guid.NewGuid();
+
+    /// <summary>
+    /// Editor-friendly name of the scene.
+    /// </summary>
+    [JsonPropertyName("name")]
+    public string Name { get; set; } = "Scene";
+
+    /// <summary>
+    /// Runtime scene ID.
+    /// </summary>
+    [JsonPropertyName("sceneId")]
+    public short SceneId { get; set; }
+
+    /// <summary>
+    /// All entities in the scene, ordered by file-local ID.
+    /// Index in this list == file-local ID.
+    /// </summary>
+    [JsonPropertyName("entities")]
+    public List<EntityData> Entities { get; set; } = new();
+}