Initial WindowGrid KWin effect for Plasma 6

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

main.cpp

@@ -0,0 +1,14 @@
+/*
+    SPDX-License-Identifier: GPL-3.0-or-later
+*/
+
+#include "windowgrid.h"
+
+namespace KWin
+{
+
+KWIN_EFFECT_FACTORY(WindowGridEffect, "metadata.json")
+
+} // namespace KWin
+
+#include "main.moc"