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>

qml/AlternatesPopup.qml

@@ -0,0 +1,54 @@
+import QtQuick
+import Sloposk
+
+// Long-press choices above a key; the one under the finger is highlighted.
+Item {
+    id: popup
+
+    required property AlternatesPicker picker
+
+    visible: picker.open
+    x: picker.rect.x
+    y: picker.rect.y
+    width: picker.rect.width
+    height: picker.rect.height
+
+    Rectangle {
+        anchors { fill: background; topMargin: 1; bottomMargin: -2 }
+        radius: background.radius
+        color: Theme.keyShadow
+    }
+
+    Rectangle {
+        id: background
+        anchors.fill: parent
+        radius: Theme.keyRadius + 2
+        color: Theme.characterKey
+    }
+
+    Row {
+        anchors.fill: parent
+
+        Repeater {
+            model: popup.picker.choices
+
+            delegate: Rectangle {
+                required property string modelData
+                required property int index
+                readonly property bool selected: index === popup.picker.selected
+
+                width: popup.width / popup.picker.choices.length
+                height: popup.height
+                radius: Theme.keyRadius
+                color: selected ? Theme.accent : "transparent"
+
+                Text {
+                    anchors.centerIn: parent
+                    text: parent.modelData
+                    font.pixelSize: parent.height * 0.4
+                    color: parent.selected ? Theme.accentText : Theme.text
+                }
+            }
+        }
+    }
+}