feat: Dear ImGui editor, GLTF materials, scene serialization

- ImGuiLayer: entity inspector (Transform/Material/Light/Camera editing),
  hierarchy panel, debug overlay with FPS graph (rlImgui-cs + ImGui.NET)
- GltfLoader.LoadWithMaterials: extracts PBR albedo, roughness, metallic,
  base color texture from glTF files
- SceneSerializer: save/load ECS world to/from JSON with custom
  Vector3/Quaternion converters, 6 tests
- ImGui disabled in tour/test mode to keep screenshots clean
- 66/66 tests passing

tests/Engine.Tests/SceneSerializerTests.cs

@@ -0,0 +1,121 @@
+using System.IO;
+using System.Numerics;
+using Engine.Core.Components;
+using Engine.Graphics;
+using Flecs.NET.Core;
+
+namespace Engine.Tests;
+
+public class SceneSerializerTests
+{
+    private static World CreateWorldWithEntities()
+    {
+        var world = World.Create();
+        world.Entity("CubeA")
+            .Set(new Transform(new Vector3(1, 2, 3), Quaternion.Identity, Vector3.One))
+            .Set(new Material(new Vector3(0.9f, 0.2f, 0.2f), 0.4f, 0.1f));
+
+        world.Entity("CubeB")
+            .Set(new Transform(new Vector3(-1, 0, 5), Quaternion.Identity, new Vector3(2, 2, 2)))
+            .Set(new Material(new Vector3(0.2f, 0.8f, 0.3f), 0.7f, 0.0f));
+
+        return world;
+    }
+
+    [Fact]
+    public void SaveToString_Produces_NonEmpty_Json()
+    {
+        using var world = CreateWorldWithEntities();
+
+        var json = SceneSerializer.SaveToString(world);
+
+        Assert.False(string.IsNullOrEmpty(json));
+        Assert.Contains("CubeA", json);
+        Assert.Contains("CubeB", json);
+    }
+
+    [Fact]
+    public void SaveToFile_Creates_File()
+    {
+        using var world = CreateWorldWithEntities();
+        var path = Path.Combine(Path.GetTempPath(), $"scene_{Guid.NewGuid():N}.json");
+
+        try
+        {
+            SceneSerializer.SaveToFile(world, path);
+
+            Assert.True(File.Exists(path));
+            var content = File.ReadAllText(path);
+            Assert.Contains("CubeA", content);
+        }
+        finally
+        {
+            if (File.Exists(path)) File.Delete(path);
+        }
+    }
+
+    [Fact]
+    public void LoadFromString_Adds_Entities()
+    {
+        using var sourceWorld = CreateWorldWithEntities();
+        var json = SceneSerializer.SaveToString(sourceWorld);
+
+        using var targetWorld = World.Create();
+        var loaded = SceneSerializer.LoadFromString(targetWorld, json);
+
+        Assert.True(loaded > 0);
+        var entity = targetWorld.Lookup("CubeA");
+        Assert.True((ulong)entity.Id != 0);
+    }
+
+    [Fact]
+    public void LoadFromFile_Restores_Entities()
+    {
+        using var sourceWorld = CreateWorldWithEntities();
+        var path = Path.Combine(Path.GetTempPath(), $"scene_{Guid.NewGuid():N}.json");
+
+        try
+        {
+            SceneSerializer.SaveToFile(sourceWorld, path);
+
+            using var targetWorld = World.Create();
+            SceneSerializer.LoadFromFile(targetWorld, path);
+
+            var entity = targetWorld.Lookup("CubeB");
+            Assert.True((ulong)entity.Id != 0);
+        }
+        finally
+        {
+            if (File.Exists(path)) File.Delete(path);
+        }
+    }
+
+    [Fact]
+    public void LoadFromFile_Throws_For_Missing_File()
+    {
+        using var world = World.Create();
+
+        Assert.Throws<FileNotFoundException>(() =>
+            SceneSerializer.LoadFromFile(world, "/nonexistent/scene.json"));
+    }
+
+    [Fact]
+    public void Roundtrip_Preserves_Transform_Position()
+    {
+        using var sourceWorld = World.Create();
+        sourceWorld.Entity("TestEntity")
+            .Set(new Transform(new Vector3(5, 10, 15), Quaternion.Identity, Vector3.One));
+
+        var json = SceneSerializer.SaveToString(sourceWorld);
+
+        using var targetWorld = World.Create();
+        SceneSerializer.LoadFromString(targetWorld, json);
+
+        var entity = targetWorld.Lookup("TestEntity");
+        Assert.True((ulong)entity.Id != 0);
+        var transform = entity.Get<Transform>();
+        Assert.Equal(5f, transform.Position.X, 0.001f);
+        Assert.Equal(10f, transform.Position.Y, 0.001f);
+        Assert.Equal(15f, transform.Position.Z, 0.001f);
+    }
+}