Gestures: flick, long-press alternates, trackpad, layer slide, rollover

- PointerTracker: per-finger gesture state machine (pure, unit-tested)
- AlternatesPicker + AlternatesPopup.qml: long-press choices, finger selects
- Space long-press trackpad moves the cursor with arrow keysyms
- Slide from .?123 onto a symbol types it and switches back
- New finger commits pending taps (iOS rollover)
- Flick animation: secondary symbol slides down as the finger moves
- Transparent popup margin with an input region that passes clicks through
- tst_keyboardsurface: end-to-end gesture tests on the real QML

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

src/input/KeyboardController.cpp

@@ -37,6 +37,7 @@ void KeyboardController::keyDown(const KeyDef &key, qint64 timeMs)
         m_engine.sendKeysym(XKB_KEY_BackSpace);
         break;
     case KeyType::LayerSwitch:
+        m_layerBeforeSwitch = m_model.layerName();
         m_model.setLayer(key.target);
         break;
     default:
@@ -81,6 +82,24 @@ void KeyboardController::keyCancelled(const KeyDef &key)
         changeShift([&] { m_shift.release(); });
 }
 
+void KeyboardController::slideReleased(const KeyDef &target, qint64 timeMs)
+{
+    keyUp(target, KeyOutput::Primary, {}, timeMs);
+    m_model.setLayer(m_layerBeforeSwitch);
+}
+
+void KeyboardController::moveCursor(QPoint steps)
+{
+    const auto repeat = [this](int count, quint32 keysym) {
+        for (int i = 0; i < count; ++i)
+            m_engine.sendKeysym(keysym);
+    };
+    repeat(qMax(0, steps.x()), XKB_KEY_Right);
+    repeat(qMax(0, -steps.x()), XKB_KEY_Left);
+    repeat(qMax(0, steps.y()), XKB_KEY_Down);
+    repeat(qMax(0, -steps.y()), XKB_KEY_Up);
+}
+
 void KeyboardController::nextLayout()
 {
     const auto &layouts = m_layouts.layouts();