feat: switch default backend to Silk.NET OpenGL

- OpenGLRenderContext: Initialize() + CreateOpenGL() for non-blocking
- OpenGLWindow: Initialize() instead of Run(), CreateGL() method
- Program.cs: default backend is now 'opengl', SwapBuffers per frame
- ImGui guarded: only initialized for RaylibRenderer, ObjectManipulator
  checks _imGuiAvailable before calling ImGui.GetIO()
- 66/66 tests, 145 FPS with OpenGL backend

src/Engine.Graphics.OpenGL/OpenGLWindow.cs

@@ -2,6 +2,7 @@ using System;
 using System.Collections.Generic;
 using Engine.Core;
 using Silk.NET.Input;
+using Silk.NET.OpenGL;
 using Silk.NET.Windowing;
 using SilkWindow = Silk.NET.Windowing.IWindow;
 
@@ -31,6 +32,8 @@ public sealed class OpenGLWindow : Engine.Core.IWindow, IDisposable
 
         _silkWindow = Window.Create(options);
         _silkWindow.Closing += () => _shouldClose = true;
+        // Initialize without Run() — enables non-blocking event loop
+        _silkWindow.Initialize();
     }
 
     public void OnLoad()
@@ -39,6 +42,8 @@ public sealed class OpenGLWindow : Engine.Core.IWindow, IDisposable
         _input.Initialize(inputContext);
     }
 
+    public GL CreateGL() => _silkWindow.CreateOpenGL();
+
     public void PumpEvents() => _silkWindow.DoEvents();
     public void Close() => _shouldClose = true;
     public string[] GetRequiredVulkanExtensions() => Array.Empty<string>();