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/OpenGLBackendRegistrar.cs

@@ -0,0 +1,16 @@
+using Engine.Graphics;
+
+namespace Engine.Graphics.OpenGL;
+
+/// <summary>
+/// Triggers registration of the OpenGL backend with the HAL factory.
+/// </summary>
+public static class OpenGLBackendRegistrar
+{
+    static OpenGLBackendRegistrar()
+    {
+        RenderBackendFactory.Register("opengl", (width, height, _) => new OpenGLRenderContext(width, height));
+    }
+
+    public static void EnsureRegistered() { }
+}