Spike: KWin input-method backend with floating layer-shell window

Registers as KWin's virtual keyboard (input-method-unstable-v1), shows a
non-focusable layer-shell overlay, and commits Unicode text. Verified in a
nested headless KWin with the textprobe tool.

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

tools/textprobe.cpp

@@ -0,0 +1,17 @@
+#include <QApplication>
+#include <QLineEdit>
+#include <cstdio>
+
+int main(int argc, char *argv[])
+{
+    QApplication app(argc, argv);
+    QLineEdit edit;
+    QObject::connect(&edit, &QLineEdit::textChanged, [](const QString &text) {
+        std::printf("TEXT:%s\n", qPrintable(text));
+        std::fflush(stdout);
+    });
+    edit.resize(400, 40);
+    edit.show();
+    edit.setFocus();
+    return app.exec();
+}