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/Key.qml

@@ -13,7 +13,9 @@ Item {
     required property real keyWidth
     required property real keyHeight
     required property bool pressed
+    required property real flickProgress
     property int shiftMode: KeyboardController.Off
+    property bool blank: false // labels hidden, e.g. in trackpad mode
 
     readonly property bool isCharacter: keyType === "char" || keyType === "space"
     readonly property bool shiftActive: keyType === "shift" && shiftMode !== KeyboardController.Off
@@ -52,13 +54,18 @@ Item {
         color: key.lit ? Theme.characterKey : Theme.functionKey
     }
 
+    // Flicking slides the secondary symbol down into the main position.
     Text {
         id: flick
-        visible: text !== ""
+        readonly property real restY: key.height * 0.1
+        readonly property real centerY: (key.height - height) / 2
+
+        visible: text !== "" && !key.blank
         text: key.flickLabel
-        anchors { horizontalCenter: parent.horizontalCenter; top: parent.top; topMargin: key.height * 0.1 }
-        font.pixelSize: key.height * 0.24
-        color: Theme.secondaryText
+        anchors.horizontalCenter: parent.horizontalCenter
+        y: restY + (centerY - restY) * key.flickProgress
+        font.pixelSize: key.height * (0.24 + 0.16 * key.flickProgress)
+        color: key.flickProgress >= 1 ? Theme.text : Theme.secondaryText
     }
 
     Text {
@@ -70,5 +77,7 @@ Item {
         }
         font.pixelSize: key.height * (key.isWord ? 0.24 : flick.visible ? 0.34 : 0.4)
         color: Theme.text
+        visible: !key.blank
+        opacity: 1 - key.flickProgress
     }
 }