wip: Silk.NET OpenGL backend — project structure, partial implementation

- Engine.Graphics.OpenGL project with Silk.NET.OpenGL, Windowing, Input
- OpenGLWindow (Silk.NET.Windowing), OpenGLInputState (Silk.NET.Input)
- OpenGLRenderer: full shader pipeline with shadow mapping, VAO/VBO upload
- OpenGLRenderContext, OpenGLBackendRegistrar
- Build errors due to Silk.NET 2.21 API differences — needs API investigation
- Raylib and Vulkan backends unchanged

src/Engine.Graphics.OpenGL/OpenGLWindow.cs

@@ -0,0 +1,64 @@
+using Engine.Core;
+using Engine.Core.Components;
+using Silk.NET.Input;
+using SilkWindow = Silk.NET.Windowing.IWindow;
+using SilkKey = Silk.NET.Input.Key;
+using SilkMouseButton = Silk.NET.Input.MouseButton;
+using EngineKey = Engine.Core.Key;
+
+namespace Engine.Graphics.OpenGL;
+
+/// <summary>
+/// Silk.NET windowing-backed implementation of IWindow.
+/// </summary>
+public sealed class OpenGLWindow : Engine.Core.IWindow, IDisposable
+{
+    private readonly SilkWindow _silkWindow;
+    private readonly OpenGLInputState _input = new();
+    private bool _shouldClose;
+    private bool _disposed;
+
+    public int Width => _silkWindow.Size.X;
+    public int Height => _silkWindow.Size.Y;
+    public bool ShouldClose => _shouldClose || _silkWindow.IsClosing;
+    IInputState Engine.Core.IWindow.Input => _input;
+    public nint Handle => 0;
+
+    public SilkWindow SilkView => _silkWindow;
+
+    public OpenGLWindow(string title, int width, int height)
+    {
+        var options = WindowOptions.Default;
+        options.Title = title;
+        options.Size = new Silk.NET.Maths.Vector2D<int>(width, height);
+        options.VSync = true;
+        options.API = new GraphicsAPI(ContextAPI.OpenGL, ContextProfile.Core, ContextFlags.Default, new APIVersion(3, 3));
+
+        _silkWindow = Silk.NET.Windowing.Window.Create(options);
+        _silkWindow.Closing += () => _shouldClose = true;
+    }
+
+    public void OnLoad()
+    {
+        var inputContext = _silkWindow.CreateInput();
+        _input.Initialize(inputContext);
+    }
+
+    public void PumpEvents()
+    {
+        _silkWindow.DoEvents();
+    }
+
+    public void Close() => _shouldClose = true;
+
+    public string[] GetRequiredVulkanExtensions() => Array.Empty<string>();
+
+    public void Dispose()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        _silkWindow.Close();
+        _silkWindow.Dispose();
+    }
+}
+