Text rules: auto-capitalization, double-space period, backspace repeat

- TextContext: field purpose/hints and text before the cursor, reported by
  the backend (KWin content_type + surrounding_text)
- TextRules: sentence-start and double-space heuristics, deletion helpers
- KeyRepeater: held backspace repeats, then deletes whole words (Ctrl+BS)
- Controller keeps a best-guess of the text before the cursor so rules work
  instantly and in fields that don't report surrounding text
- No auto-capitalization or smart punctuation in password/URL/email fields

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

src/backend/KWinInputMethod.cpp

@@ -37,14 +37,9 @@ bool KWinInputMethod::isActive() const
     return m_context != nullptr;
 }
 
-QString KWinInputMethod::surroundingText() const
+TextContext KWinInputMethod::context() const
 {
-    return m_context ? m_context->surroundingText() : QString();
-}
-
-int KWinInputMethod::cursorPosition() const
-{
-    return m_context ? m_context->cursorPosition() : -1;
+    return m_context ? m_context->context() : TextContext();
 }
 
 void KWinInputMethod::commitText(const QString &text)
@@ -68,7 +63,7 @@ void KWinInputMethod::sendKeysym(quint32 keysym, Qt::KeyboardModifiers 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);
+    connect(m_context.get(), &KWinInputContext::contextChanged, this, &TextBackend::contextChanged);
     Q_EMIT activeChanged();
 }
 
@@ -76,5 +71,5 @@ void KWinInputMethod::deactivate()
 {
     m_context.reset();
     Q_EMIT activeChanged();
-    Q_EMIT surroundingChanged();
+    Q_EMIT contextChanged();
 }