Hold Keys mode: keys stay down while touched, like a hardware keyboard

- HardwareKeys presses a key's unshifted keysym on touch and releases it
  on lift, through fake input; held keys repeat and combine as on a
  physical keyboard (games, Shift+click)
- Shift/Ctrl/Alt/Super are plain held keys, caps lock is the system's,
  dead keys compose in the app
- No long-press, flick, trackpad or rollover in this mode; nothing is
  left held when the mode is turned off or touches are cancelled
- Settings switch; default stays the touch (virtual keyboard) mode

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

tests/FakeEngine.h

@@ -3,6 +3,7 @@
 #include "engine/InputEngine.h"
 
 #include <QList>
+#include <QSet>
 #include <QString>
 
 // Records what the keyboard sends and applies it to a simulated field, so
@@ -27,8 +28,25 @@ public:
         lastModifiers = modifiers;
     }
 
+    void setKeyPressed(quint32 keysym, bool pressed) override
+    {
+        keyEvents.append({keysym, pressed});
+        if (pressed)
+            held.insert(keysym);
+        else
+            held.remove(keysym);
+    }
+
+    struct KeyEvent {
+        quint32 keysym;
+        bool pressed;
+        bool operator==(const KeyEvent &) const = default;
+    };
+
     QString output;               // the field's text
     int mismatchedReplacements = 0; // replaced text that wasn't before the cursor
     QList<quint32> keysyms;
     Qt::KeyboardModifiers lastModifiers;
+    QList<KeyEvent> keyEvents; // setKeyPressed calls
+    QSet<quint32> held;
 };