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/Main.qml

@@ -8,17 +8,29 @@ Window {
     required property KeyboardController controller
     property var placement: null // FloatingWindow, set from C++
 
+    // Transparent room above the keyboard for long-press popups on the top row.
+    readonly property real popupMargin: 90
+    // Outside this, input passes through to whatever is below the window.
+    readonly property rect inputRegion: keyboard.alternates.open
+        ? Qt.rect(0, 0, width, height)
+        : Qt.rect(panel.x, panel.y, panel.width, panel.height)
+
     width: 1000
-    height: 340
+    height: 340 + popupMargin
     visible: false
     color: "transparent"
 
+    onInputRegionChanged: placement?.setInputRegion(inputRegion)
+    onPlacementChanged: placement?.setInputRegion(inputRegion)
+
     Rectangle {
-        anchors.fill: parent
+        id: panel
+        anchors { fill: parent; topMargin: root.popupMargin }
         radius: Theme.windowRadius
         color: Theme.background
 
         Keyboard {
+            id: keyboard
             anchors { left: parent.left; right: parent.right; top: parent.top; bottom: handle.top }
             anchors.margins: 2
             model: root.layoutModel