Dead keys: act on compose status at press time

- Two dead keys that compose (´´ → ´) type the result right away; a dead
  key that can't continue the sequence types the old accents and starts anew
- With Ctrl/Alt/Super, a dead key sends its dead keysym, as the physical
  key does, not the spacing accent from its label

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

tests/tst_controller.cpp

@@ -373,6 +373,16 @@ private Q_SLOTS:
         QCOMPARE(engine.output, u"+Ľ2");
     }
 
+    void ctrlWithDeadKeySendsTheDeadKeysym()
+    {
+        KeyDef acute = makeKey(KeyType::Character, u"´"_s);
+        acute.dead = XKB_KEY_dead_acute;
+        tap(modifierKey(u"ctrl"_s));
+        tap(acute);
+        QCOMPARE(engine.keysyms, QList<quint32>{XKB_KEY_dead_acute});
+        QCOMPARE(engine.lastModifiers, Qt::ControlModifier);
+    }
+
     void deadKeyComposesWithNextCharacter()
     {
         KeyDef acute = makeKey(KeyType::Character, u"´"_s);