gui: fix Wayland SIGSEGV on launch; guard stderr for windowed bundles

e15329a's window-activation code crashed the GUI at startup on Wayland:
raise_()/activateWindow()/move() go through Qt-Wayland's xdg restack/
xdg-activation paths, which SEGSEGV in PySide6 6.9 as shipped on Guix
(faulthandler pinpointed win.raise_(); QT_QPA_PLATFORM=wayland reproduces,
--selftest — plain show() — never crashed). Gate the whole center/raise/
activate block behind platformName() == "xcb": on X11 it is the canonical
show-then-activate sequence and verified working under XWayland; on Wayland
the compositor maps and focuses the new toplevel itself (raise/activate were
no-ops there even before they crashed). Strict equality keeps wayland-egl
and any unknown platform on the safe plain-show path. Centering is computed
before show() again, as pre-e15329a, so X11 windows map already-centered.

Also make every sys.stderr access None-safe: PyInstaller --windowed (>=5.8,
i.e. the shipped Windows .exe) sets sys.stderr = None, so the unguarded
startup notice would have raised AttributeError right after show() and
killed the window; a dead pipe similarly raises on flush. Guard the notice
(+ try/except OSError), the import-error message, and the debug discovery
echo. Verified: launch stays alive on wayland, xcb, and offscreen-with-
fd2-closed; --selftest passes offscreen/wayland incl. VAPTVUPT_DEBUG=1
with stderr closed.

gui/src/zupt_gui.py

@@ -38,6 +38,7 @@ except ImportError:
         from PyQt6.QtGui import QPalette, QColor, QIcon, QPixmap
         QT_BINDING = "PyQt6"
     except ImportError:
+        if sys.stderr is not None:   # None under PyInstaller --windowed
             sys.stderr.write(
                 "ERROR: vaptvupt-gui requires PySide6 or PyQt6. Install one of:\n"
                 "  Debian/Ubuntu:  sudo apt install python3-pyqt6\n"
@@ -64,7 +65,8 @@ _DISCOVERY_LOG = []
 def _discovery_log(msg):
     _DISCOVERY_LOG.append(msg)
     # Echo to stderr if VAPTVUPT_DEBUG or ZUPT_DEBUG is set
-    if os.environ.get("VAPTVUPT_DEBUG") or os.environ.get("ZUPT_DEBUG"):
+    if ((os.environ.get("VAPTVUPT_DEBUG") or os.environ.get("ZUPT_DEBUG"))
+            and sys.stderr is not None):   # None under PyInstaller --windowed
         sys.stderr.write(f"  [discovery] {msg}\n")
 
 def _is_runnable(path):
@@ -908,24 +910,39 @@ def main():
               f"event loop ran (rc={rc}); CLI={VAPTVUPT}")
         return rc
 
-    # Center on the active screen and force focus. Without this the window can
-    # open off-screen or behind the last-focused frame on a tiling compositor
-    # (Sway/i3/Hyprland) — the usual cause of "the GUI won't start / is stuck":
-    # it launched, but you can't see it.
+    # Center + raise + focus ONLY on X11 (xcb), where a stacking WM may place
+    # the window off-screen or leave it unfocused. On Wayland the compositor
+    # owns placement and focus, and these calls (self-move / xdg restack /
+    # xdg-activation) SEGFAULT some Qt-Wayland builds — including PySide6 6.9 as
+    # shipped on Guix — so they must not run there. Plain show() is what
+    # --selftest exercises and is stable; the compositor maps and focuses the
+    # new toplevel itself. On Windows/macOS Qt's automatic placement centers
+    # first windows and the OS foregrounds a freshly launched app, so skipping
+    # is safe there too. Strict == "xcb" keeps wayland-egl etc. on the safe path.
+    is_x11 = app.platformName() == "xcb"
+    if is_x11:
         scr = app.primaryScreen()
         if scr is not None:
             fg = win.frameGeometry()
             fg.moveCenter(scr.availableGeometry().center())
             win.move(fg.topLeft())
     win.show()
+    if is_x11:
         win.raise_()
         win.activateWindow()
 
     # A GUI blocks the launching shell, so a working launch otherwise looks like
-    # a "stuck" terminal. Emit one line to stderr so it's unambiguous.
-    sys.stderr.write(f"VaptVupt {ZUPT_VER_NUMBER} GUI started — window open "
-                     f"(close it to exit).\n")
+    # a "stuck" terminal. Emit one line to stderr so it's unambiguous. Guarded:
+    # PyInstaller --windowed sets sys.stderr to None (any write would raise and
+    # kill the window we just showed), and a dead pipe raises OSError on flush —
+    # a courtesy notice must never take the GUI down.
+    if sys.stderr is not None:
+        try:
+            sys.stderr.write(f"VaptVupt {ZUPT_VER_NUMBER} GUI started — "
+                             f"window open (close it to exit).\n")
             sys.stderr.flush()
+        except OSError:
+            pass
     return app.exec()
 
 if __name__ == "__main__":