Adds packaging/guix/vaptvupt.scm so VaptVupt is reproducibly installable on
GNU Guix (guix package -f). The GUI launcher sets LD_LIBRARY_PATH to the Qt6
leaf libraries PySide6 needs but does not carry in its RUNPATH — libGL (mesa),
libxkbcommon, the X11/xcb family, libzstd (from zstd's "lib" output), harfbuzz,
icu, wayland, glib, dbus, ... Without them, `import PySide6.QtWidgets` fails with
"libGL.so.1 / libzstd.so.1: cannot open shared object file" and the GUI prints
"requires PySide6 or PyQt6" on Guix (which has no global /usr/lib to satisfy
them). Qt's own libraries are deliberately excluded from LD_LIBRARY_PATH so they
resolve via PySide6's RUNPATH (a second copy causes private-API symbol clashes).
Also puts python-shiboken-6's site-packages on GUIX_PYTHONPATH.
Verified: full PySide6 QtWidgets/QtGui/QtCore import + QApplication, and the
windowed GUI launches, on this Guix system.
