feat: add scene serialization (JSON + binary) with import pipeline

Implement scene save/load for editor and runtime.
Editor JSON (.gscene) uses Utf8JsonWriter for inline component objects.
Runtime binary (.imported) stores marshalled component data with
entity field offset metadata for AOT-safe remapping.

- SceneSerializationService: save from EditorWorld, load into EditorWorld
- SceneAsset + SceneAssetHandler: .gscene import/pack pipeline
- AssetManager.Scene + SceneLoader: runtime binary deserialization
- Scene: [JsonConstructor] + [JsonIgnore] for round-trip
- Component: GetComponentIDByName for editor-side type lookup
- 10 unit tests (save, load, round-trip, unknown comp, invalid version)

Also guard DSLShaderCompiler editor code with #if GHOST_EDITOR,
add GC.SuppressFinalize to EditorWorldService, and switch Archetype
debug fields from #if GHOST_EDITOR to #if DEBUG.

src/Runtime/Ghost.Engine/Core/Scene.cs

@@ -1,6 +1,7 @@
 using Ghost.Entities;
 using Misaki.HighPerformance.LowLevel.Buffer;
 using Misaki.HighPerformance.LowLevel.Collections;
+using System.Text.Json.Serialization;
 
 namespace Ghost.Engine.Core;
 
@@ -19,6 +20,7 @@ public readonly struct Scene : IEquatable<Scene>
     /// <summary>
     /// Gets whether this scene is valid.
     /// </summary>
+    [JsonIgnore]
     public bool IsValid => _id >= 0;
 
     /// <summary>
@@ -26,7 +28,8 @@ public readonly struct Scene : IEquatable<Scene>
     /// </summary>
     public static Scene Invalid => new(-1);
 
-    internal Scene(short id)
+    [JsonConstructor]
+    public Scene(short id)
     {
         _id = id;
     }