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/layout/LayoutModel.cpp

@@ -50,6 +50,7 @@ QVariant LayoutModel::data(const QModelIndex &index, int role) const
     case KeyWidthRole: return rect.width();
     case KeyHeightRole: return rect.height();
     case PressedRole: return m_pressCount.at(index.row()) > 0;
+    case FlickProgressRole: return m_flickProgress.at(index.row());
     }
     return {};
 }
@@ -65,6 +66,7 @@ QHash<int, QByteArray> LayoutModel::roleNames() const
         {KeyWidthRole, "keyWidth"},
         {KeyHeightRole, "keyHeight"},
         {PressedRole, "pressed"},
+        {FlickProgressRole, "flickProgress"},
     };
 }
 
@@ -115,6 +117,14 @@ void LayoutModel::setPressed(int row, bool pressed)
         Q_EMIT dataChanged(index(row), index(row), {PressedRole});
 }
 
+void LayoutModel::setFlickProgress(int row, qreal progress)
+{
+    if (row < 0 || row >= m_flickProgress.size() || qFuzzyCompare(m_flickProgress[row], progress))
+        return;
+    m_flickProgress[row] = progress;
+    Q_EMIT dataChanged(index(row), index(row), {FlickProgressRole});
+}
+
 int LayoutModel::keyAt(QPointF pos) const
 {
     int nearest = -1;
@@ -142,6 +152,7 @@ void LayoutModel::rebuild()
         }
     }
     m_pressCount = QList<int>(m_keys.size(), 0);
+    m_flickProgress = QList<qreal>(m_keys.size(), 0.0);
     relayout();
     endResetModel();
 }