ui: the sidebar is two tabs, and a toggle disables its pane

Replace the sidebar's five-panel scrolling column with an AdwViewStack
carrying two pages — Measure (cursors + measurements) and Signals
(trigger + math + channel list) — with an AdwViewSwitcher above it. The
trigger and cursors toggles change from "reveal/hide my pane" to
"enable/disable my feature, leaving the pane insensitive", following the
disable-not-hide principle. The keys probe loses a duplicate registration
that was toggling the sidebar back on before the exit probe captured its
state; the exit-probe captures now read adw_overlay_split_view_get_show_
sidebar() instead of gtk_widget_get_visible(), which AdwOverlaySplitView
never changes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui_signals.cpp

@@ -137,6 +137,7 @@ void on_signal_color_chosen(GObject* source, GAsyncResult* res, gpointer user) {
 /// buttons made every row about twice this tall, which is what turned a four-signal
 /// stream into a panel you have to scroll.
 void rebuild_signal_list(App& app, const std::vector<SignalInfo>& signals) {
+  if (app.signal_list == nullptr) return;
   app.updating_panels = true;
   GtkWidget* list = app.signal_list;
   GtkWidget* child = gtk_widget_get_first_child(list);
@@ -300,12 +301,6 @@ void rebuild_signal_list(App& app, const std::vector<SignalInfo>& signals) {
   app.updating_panels = false;
 }
 
-void on_signals_toggled(GtkToggleButton* button, gpointer user) {
-  App* app = static_cast<App*>(user);
-  gtk_revealer_set_reveal_child(GTK_REVEALER(app->signal_revealer),
-                                gtk_toggle_button_get_active(button));
-}
-
 gboolean on_drop_channel(GtkDropTarget*, const GValue* value, double x, double y,
                          gpointer user) {
   App* app = static_cast<App*>(user);
@@ -396,17 +391,9 @@ void rebuild_signal_panels(App& app) {
   rebuild_trigger_signals(app);
   refresh_window_limits(app);
 
-  // Nothing to configure before a CONFIG arrives: hide the whole column and its
-  // toolbar buttons rather than showing two empty panels. The first CONFIG
-  // reveals the signal panel (the trigger panel stays behind its own button).
+  // Nothing to configure before a CONFIG arrives: hide the whole column.
   update_side_visibility(app);
-  if (have_signals && !app.signals_panel_initialised) {
-    app.signals_panel_initialised = true;
-    gtk_toggle_button_set_active(GTK_TOGGLE_BUTTON(app.signal_toggle), TRUE);
-  }
   if (!have_signals) {
-    app.signals_panel_initialised = false;
-    gtk_toggle_button_set_active(GTK_TOGGLE_BUTTON(app.signal_toggle), FALSE);
     gtk_toggle_button_set_active(GTK_TOGGLE_BUTTON(app.trigger_toggle), FALSE);
   }
   app.signal_generation = app.model->generation();