Add simple scene graph

Ghost.Editor.Core/Serializer/Converters/EntityJsonConverter.cs

@@ -0,0 +1,115 @@
+using Ghost.Entities;
+using Ghost.Engine.IO;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Ghost.Editor.Core.Serializer.Converters;
+
+/// <summary>
+/// JSON converter for Entity that handles automatic ID remapping during deserialization.
+/// </summary>
+/// <remarks>
+/// During serialization, writes the file-local entity ID.
+/// During deserialization, reads the file-local ID and translates it to the runtime Entity
+/// using the active SerializationContext.
+/// </remarks>
+public class EntityJsonConverter : JsonConverter<Entity>
+{
+    public override Entity Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+    {
+        if (reader.TokenType == JsonTokenType.Null)
+        {
+            return Entity.Invalid;
+        }
+
+        if (reader.TokenType != JsonTokenType.StartObject)
+        {
+            throw new JsonException("Expected StartObject token for Entity.");
+        }
+
+        int fileId = -1;
+        int generation = -1;
+
+        while (reader.Read())
+        {
+            if (reader.TokenType == JsonTokenType.EndObject)
+            {
+                break;
+            }
+
+            if (reader.TokenType == JsonTokenType.PropertyName)
+            {
+                var propertyName = reader.GetString();
+                reader.Read();
+
+                switch (propertyName)
+                {
+                    case "ID":
+                    case "id":
+                        fileId = reader.GetInt32();
+                        break;
+                    case "Generation":
+                    case "generation":
+                        generation = reader.GetInt32();
+                        break;
+                }
+            }
+        }
+
+        if (fileId == Entity.INVALID_ID)
+        {
+            return Entity.Invalid;
+        }
+
+        // If we have a serialization context, remap the file ID to runtime entity
+        var context = SerializationContext.Current;
+        if (context != null)
+        {
+            if (context.TryGetEntity(fileId, out var runtimeEntity))
+            {
+                return runtimeEntity;
+            }
+
+            // If entity not found in map, return invalid
+            return Entity.Invalid;
+        }
+
+        // No context means we're not in a deserialization scope - should not happen
+        throw new InvalidOperationException("Entity deserialization requires an active SerializationContext.");
+    }
+
+    public override void Write(Utf8JsonWriter writer, Entity value, JsonSerializerOptions options)
+    {
+        if (!value.IsValid)
+        {
+            writer.WriteNullValue();
+            return;
+        }
+
+        writer.WriteStartObject();
+
+        // If we have a serialization context, write the file-local ID
+        var context = SerializationContext.Current;
+        if (context != null)
+        {
+            if (context.TryGetFileId(value, out var fileId))
+            {
+                writer.WriteNumber("ID", fileId);
+            }
+            else
+            {
+                // Entity not in context - register it now
+                var newFileId = context.RegisterEntityForSerialization(value);
+                writer.WriteNumber("ID", newFileId);
+            }
+        }
+        else
+        {
+            // No context - write the runtime ID (for debugging or non-scene serialization)
+            writer.WriteNumber("ID", value.ID);
+        }
+
+        writer.WriteNumber("Generation", value.Generation);
+        writer.WriteEndObject();
+    }
+}