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>

tests/tst_controller.cpp

@@ -1,6 +1,5 @@
 #include "engine/InputEngine.h"
 #include "input/KeyboardController.h"
-#include "layout/LayoutLoader.h"
 #include "layout/LayoutModel.h"
 #include "layout/LayoutRepository.h"
 
@@ -106,6 +105,22 @@ private Q_SLOTS:
         QCOMPARE(model.layerName(), u"numbers");
     }
 
+    void slideTypesSymbolAndSwitchesBack()
+    {
+        KeyDef layer = makeKey(KeyType::LayerSwitch);
+        layer.target = u"numbers"_s;
+        controller->keyDown(layer, 0);
+        controller->slideReleased(makeKey(KeyType::Character, u"5"_s), 100);
+        QCOMPARE(engine.output, u"5");
+        QCOMPARE(model.layerName(), u"letters");
+    }
+
+    void trackpadMovesCursor()
+    {
+        controller->moveCursor({2, -1});
+        QCOMPARE(engine.keysyms, (QList<quint32>{XKB_KEY_Right, XKB_KEY_Right, XKB_KEY_Up}));
+    }
+
     void globeCyclesLayouts()
     {
         QCOMPARE(model.layoutName(), u"English (US)");