Hold Keys mode: keys stay down while touched, like a hardware keyboard

- HardwareKeys presses a key's unshifted keysym on touch and releases it
  on lift, through fake input; held keys repeat and combine as on a
  physical keyboard (games, Shift+click)
- Shift/Ctrl/Alt/Super are plain held keys, caps lock is the system's,
  dead keys compose in the app
- No long-press, flick, trackpad or rollover in this mode; nothing is
  left held when the mode is turned off or touches are cancelled
- Settings switch; default stays the touch (virtual keyboard) mode

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/CMakeLists.txt

@@ -8,6 +8,7 @@ qt_add_library(sloposk_core STATIC
     input/ShiftState.h input/ShiftState.cpp
     input/Modifiers.h input/Modifiers.cpp
     input/DeadKeys.h input/DeadKeys.cpp
+    input/HardwareKeys.h input/HardwareKeys.cpp
     input/PointerTracker.h input/PointerTracker.cpp
     input/AlternatesPicker.h input/AlternatesPicker.cpp
     input/KeyRepeater.h input/KeyRepeater.cpp