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
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