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/IdRemapTable.cs

@@ -0,0 +1,148 @@
+using System.Collections.ObjectModel;
+using Ghost.Entities;
+
+namespace Ghost.Editor.Core.SceneGraph.Serialization;
+
+/// <summary>
+/// Maps file-local entity IDs to global entity IDs.
+/// Used when loading scenes to remap entity references from file-local IDs to runtime global IDs.
+/// </summary>
+public class IdRemapTable
+{
+    /// <summary>
+    /// Maps file-local ID (index) to global Entity ID.
+    /// </summary>
+    private readonly Dictionary<int, Entity> _localToGlobal;
+
+    /// <summary>
+    /// Maps global Entity ID to file-local ID.
+    /// </summary>
+    private readonly Dictionary<Entity, int> _globalToLocal;
+
+    public IdRemapTable()
+    {
+        _localToGlobal = new Dictionary<int, Entity>();
+        _globalToLocal = new Dictionary<Entity, int>();
+    }
+
+    /// <summary>
+    /// Registers the mapping between a file-local ID and global entity ID.
+    /// </summary>
+    public void Register(int fileLocalId, Entity globalEntityId)
+    {
+        if (fileLocalId < 0)
+            throw new ArgumentException("File-local ID must be >= 0", nameof(fileLocalId));
+
+        _localToGlobal[fileLocalId] = globalEntityId;
+        _globalToLocal[globalEntityId] = fileLocalId;
+    }
+
+    /// <summary>
+    /// Gets the global entity ID for a file-local ID.
+    /// Returns Entity.Invalid if not found.
+    /// </summary>
+    public Entity GetGlobalId(int fileLocalId)
+    {
+        _localToGlobal.TryGetValue(fileLocalId, out var globalId);
+        return globalId;
+    }
+
+    /// <summary>
+    /// Gets the file-local ID for a global entity ID.
+    /// Returns -1 if not found.
+    /// </summary>
+    public int GetLocalId(Entity globalEntityId)
+    {
+        return _globalToLocal.TryGetValue(globalEntityId, out var localId) ? localId : -1;
+    }
+
+    /// <summary>
+    /// Remaps an entity reference from file-local ID to global ID.
+    /// Throws if the file-local ID is not registered.
+    /// </summary>
+    public Entity RemapReference(int fileLocalId)
+    {
+        if (!_localToGlobal.TryGetValue(fileLocalId, out var globalId))
+        {
+            throw new KeyNotFoundException($"File-local entity ID {fileLocalId} not found in remap table.");
+        }
+
+        return globalId;
+    }
+
+    /// <summary>
+    /// Gets the count of mapped entities.
+    /// </summary>
+    public int Count => _localToGlobal.Count;
+
+    /// <summary>
+    /// Gets all mapped local->global pairs.
+    /// </summary>
+    public IEnumerable<KeyValuePair<int, Entity>> GetMappings()
+    {
+        return _localToGlobal.AsEnumerable();
+    }
+}
+
+/// <summary>
+/// Contains context information for loading or saving a scene.
+/// Includes component type information and entity remapping logic.
+/// </summary>
+public class SceneSerializationContext
+{
+    /// <summary>
+    /// Maps file-local entity IDs to runtime global entity IDs.
+    /// </summary>
+    public IdRemapTable IdRemap { get; }
+
+    /// <summary>
+    /// Scene ID being serialized/deserialized.
+    /// </summary>
+    public short SceneId { get; }
+
+    /// <summary>
+    /// Editor world where entities are being loaded/saved.
+    /// </summary>
+    public World EditorWorld { get; }
+
+    /// <summary>
+    /// List of entities in the order they appear in the saved file.
+    /// Index corresponds to file-local ID.
+    /// </summary>
+    public List<Entity> EntityOrder { get; }
+
+    /// <summary>
+    /// Validation errors encountered during serialization.
+    /// </summary>
+    public List<string> ValidationErrors { get; }
+
+    public SceneSerializationContext(short sceneId, World editorWorld)
+    {
+        SceneId = sceneId;
+        EditorWorld = editorWorld ?? throw new ArgumentNullException(nameof(editorWorld));
+        IdRemap = new IdRemapTable();
+        EntityOrder = new List<Entity>();
+        ValidationErrors = new List<string>();
+    }
+
+    /// <summary>
+    /// Adds a validation error message.
+    /// </summary>
+    public void AddValidationError(string message)
+    {
+        ValidationErrors.Add(message);
+    }
+
+    /// <summary>
+    /// Returns true if there are any validation errors.
+    /// </summary>
+    public bool HasErrors => ValidationErrors.Count > 0;
+
+    /// <summary>
+    /// Gets all validation errors as a single string.
+    /// </summary>
+    public string GetErrorsSummary()
+    {
+        return string.Join("\n", ValidationErrors);
+    }
+}