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_alternatespicker.cpp

@@ -0,0 +1,52 @@
+#include "input/AlternatesPicker.h"
+
+#include <QTest>
+
+using namespace Qt::StringLiterals;
+
+class TestAlternatesPicker : public QObject
+{
+    Q_OBJECT
+
+private Q_SLOTS:
+    void opensAboveKeyExtendingRight()
+    {
+        AlternatesPicker picker;
+        picker.open(QRectF(100, 100, 50, 60), {u"é"_s, u"è"_s, u"ê"_s}, 1000, 8);
+        QVERIFY(picker.isOpen());
+        QCOMPARE(picker.rect(), QRectF(100, 32, 150, 60));
+        QCOMPARE(picker.selectedText(), u"é");
+    }
+
+    void staysInsideRightEdgeAndSelectsUnderFinger()
+    {
+        AlternatesPicker picker;
+        picker.open(QRectF(900, 100, 50, 60), {u"a"_s, u"b"_s, u"c"_s}, 1000, 8);
+        QCOMPARE(picker.rect().left(), 850.0);
+        QCOMPARE(picker.selectedText(), u"b"); // key center x=925 lies over the second cell
+    }
+
+    void selectionFollowsFingerAndClamps()
+    {
+        AlternatesPicker picker;
+        picker.open(QRectF(100, 100, 50, 60), {u"a"_s, u"b"_s, u"c"_s}, 1000, 8);
+        picker.selectAt(210);
+        QCOMPARE(picker.selected(), 2);
+        picker.selectAt(-50);
+        QCOMPARE(picker.selected(), 0);
+        picker.selectAt(5000);
+        QCOMPARE(picker.selected(), 2);
+    }
+
+    void closeClears()
+    {
+        AlternatesPicker picker;
+        picker.open(QRectF(0, 0, 10, 10), {u"a"_s}, 100, 0);
+        picker.close();
+        QVERIFY(!picker.isOpen());
+        QVERIFY(picker.selectedText().isEmpty());
+    }
+};
+
+QTEST_GUILESS_MAIN(TestAlternatesPicker)
+#include "tst_alternatespicker.moc"