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/Keyboard.qml

@@ -8,6 +8,12 @@ KeyboardSurface {
         model: surface.model
         delegate: Key {
             shiftMode: surface.controller ? surface.controller.shiftMode : KeyboardController.Off
+            blank: surface.trackpadActive
         }
     }
+
+    AlternatesPopup {
+        picker: surface.alternates
+        z: 1
+    }
 }