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>

src/window/FloatingWindow.h

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <QObject>
+#include <QPoint>
+
+class QWindow;
+namespace LayerShellQt { class Window; }
+
+// Places a keyboard window as a free-floating overlay that never takes focus.
+// On Wayland: a layer-shell surface anchored top-left, positioned via margins.
+// On X11: a frameless, non-focusable, always-on-top tool window.
+class FloatingWindow : public QObject
+{
+    Q_OBJECT
+    Q_PROPERTY(QPoint position READ position WRITE setPosition NOTIFY positionChanged)
+
+public:
+    // Must be called before the window is shown for the first time.
+    explicit FloatingWindow(QWindow *window, QObject *parent = nullptr);
+
+    QPoint position() const { return m_position; }
+    void setPosition(QPoint position);
+
+    Q_INVOKABLE void moveBy(qreal dx, qreal dy);
+    Q_INVOKABLE void show();
+    Q_INVOKABLE void hide();
+
+Q_SIGNALS:
+    void positionChanged();
+
+private:
+    QPoint clamped(QPoint position) const;
+    QPoint defaultPosition() const;
+    void apply();
+
+    QWindow *m_window;
+    LayerShellQt::Window *m_layerSurface = nullptr;
+    QPoint m_position;
+};