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/backend/TextBackend.h

@@ -0,0 +1,33 @@
+#pragma once
+
+#include <QObject>
+#include <QString>
+
+// Delivers text produced by the keyboard to the focused application.
+// Implementations hide the transport (KWin input method, uinput, XTest, ...).
+// Keys are identified by XKB keysyms so every backend speaks the same language.
+class TextBackend : public QObject
+{
+    Q_OBJECT
+    Q_PROPERTY(bool active READ isActive NOTIFY activeChanged)
+    Q_PROPERTY(QString surroundingText READ surroundingText NOTIFY surroundingChanged)
+    Q_PROPERTY(int cursorPosition READ cursorPosition NOTIFY surroundingChanged)
+
+public:
+    using QObject::QObject;
+
+    // True while a text field is focused and accepts input.
+    virtual bool isActive() const = 0;
+
+    // Text around the cursor, if the backend knows it (empty otherwise).
+    virtual QString surroundingText() const { return {}; }
+    virtual int cursorPosition() const { return -1; }
+
+    Q_INVOKABLE virtual void commitText(const QString &text) = 0;
+    Q_INVOKABLE virtual void setPreedit(const QString &text) = 0;
+    Q_INVOKABLE virtual void sendKeysym(quint32 keysym, Qt::KeyboardModifiers modifiers = Qt::NoModifier) = 0;
+
+Q_SIGNALS:
+    void activeChanged();
+    void surroundingChanged();
+};