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/KWinInputMethod.cpp

@@ -0,0 +1,80 @@
+#include "KWinInputMethod.h"
+#include "KWinInputContext.h"
+
+KWinInputMethodGlobal::KWinInputMethodGlobal()
+    : QWaylandClientExtensionTemplate(1)
+{
+    initialize();
+}
+
+void KWinInputMethodGlobal::zwp_input_method_v1_activate(::zwp_input_method_context_v1 *context)
+{
+    Q_EMIT activated(context);
+}
+
+void KWinInputMethodGlobal::zwp_input_method_v1_deactivate(::zwp_input_method_context_v1 *)
+{
+    Q_EMIT deactivated();
+}
+
+KWinInputMethod::KWinInputMethod(QObject *parent)
+    : TextBackend(parent)
+{
+    connect(&m_global, &KWinInputMethodGlobal::activated, this, &KWinInputMethod::activate);
+    connect(&m_global, &KWinInputMethodGlobal::deactivated, this, &KWinInputMethod::deactivate);
+    connect(&m_global, &QWaylandClientExtension::activeChanged, this, &KWinInputMethod::availableChanged);
+}
+
+KWinInputMethod::~KWinInputMethod() = default;
+
+bool KWinInputMethod::isAvailable() const
+{
+    return m_global.isActive();
+}
+
+bool KWinInputMethod::isActive() const
+{
+    return m_context != nullptr;
+}
+
+QString KWinInputMethod::surroundingText() const
+{
+    return m_context ? m_context->surroundingText() : QString();
+}
+
+int KWinInputMethod::cursorPosition() const
+{
+    return m_context ? m_context->cursorPosition() : -1;
+}
+
+void KWinInputMethod::commitText(const QString &text)
+{
+    if (m_context)
+        m_context->commit(text);
+}
+
+void KWinInputMethod::setPreedit(const QString &text)
+{
+    if (m_context)
+        m_context->preedit(text);
+}
+
+void KWinInputMethod::sendKeysym(quint32 keysym, Qt::KeyboardModifiers modifiers)
+{
+    if (m_context)
+        m_context->sendKeysym(keysym, modifiers);
+}
+
+void KWinInputMethod::activate(::zwp_input_method_context_v1 *context)
+{
+    m_context = std::make_unique<KWinInputContext>(context);
+    connect(m_context.get(), &KWinInputContext::surroundingChanged, this, &TextBackend::surroundingChanged);
+    Q_EMIT activeChanged();
+}
+
+void KWinInputMethod::deactivate()
+{
+    m_context.reset();
+    Q_EMIT activeChanged();
+    Q_EMIT surroundingChanged();
+}