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/window/FloatingWindow.cpp

@@ -51,6 +51,11 @@ void FloatingWindow::moveBy(qreal dx, qreal dy)
     setPosition(m_position + QPoint(qRound(dx), qRound(dy)));
 }
 
+void FloatingWindow::setInputRegion(const QRectF &region)
+{
+    m_window->setMask(QRegion(region.toAlignedRect()));
+}
+
 void FloatingWindow::show()
 {
     m_window->setVisible(true);