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>

src/CMakeLists.txt

@@ -5,6 +5,8 @@ qt_add_library(sloposk_core STATIC
     engine/InputEngine.h
     engine/DirectEngine.h engine/DirectEngine.cpp
     input/ShiftState.h input/ShiftState.cpp
+    input/PointerTracker.h input/PointerTracker.cpp
+    input/AlternatesPicker.h input/AlternatesPicker.cpp
     input/KeyboardController.h input/KeyboardController.cpp
     input/KeyboardSurface.h input/KeyboardSurface.cpp
     layout/Layout.h
@@ -19,6 +21,7 @@ set(SLOPOSK_QML_FILES
     Main.qml
     Keyboard.qml
     Key.qml
+    AlternatesPopup.qml
     DragHandle.qml
     Theme.qml
 )