fix: the guided tour selects the page a stop's target lives on

A stop whose target is on another page of a stack was entered without
switching to that page, so the spotlight aimed at a widget that was not
on screen. A stop now declares the page it stands on (`TourStop::page`)
and the tour selects it as the step is entered -- the window's trigger
and signals stops put the sidebar on Signals, the chooser's tabs stop
shows the Waveforms page and stepping off it brings Sources back.

The stop filter moved into `tour_begin()` and now runs on
`gtk_widget_get_mapped()`: GTK 4.24's `is_visible()` ignores
`child_visible`, so it kept a stop on another page *and* one in a
collapsed pane alike. A wrong page is kept and selected; a collapsed
pane, whose stack is unmapped too, is still dropped.

src/ui.cpp

@@ -5808,6 +5808,9 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
   if (const char* env = g_getenv("UDPSCOPE_PROBE_HELPUI_MS")) {
     helpui_probe_next(app, 0, static_cast<guint>(std::atoi(env)));
   }
+  if (const char* env = g_getenv("UDPSCOPE_PROBE_TOUR_SURFACE_MS")) {
+    tour_surface_probe_next(app, static_cast<guint>(std::atoi(env)));
+  }
   if (const char* env = g_getenv("UDPSCOPE_PROBE_CONFIGS_MS")) {
     g_timeout_add(static_cast<guint>(std::atoi(env)), configs_probe, &app);
   }