signal page: the header names it, numbered; no duplicate title or back button

The slide-over's header already prints the open page's title beside its Close button, so the
signal page's body carried the name twice and a "Back to signals" button that only repeated
navigation the header owns. Both are gone.

The header's title is now the **numbered** name -- `<n>. <name>`, `measure_channel_number()`'s
number, the same one the signal badges and the measurements pane use -- so the panel says
*which acquired channel* it is and not only what it is called. `slideover_set_title()`
retitles a page in place (the open one included): the number follows the stream's channel
numbering, and the page is refilled whenever the model's signal list changes.

Probe: `[signalpage]` reads `title='1. sine' title_ok=1` (the header text against the numbered
name) and `no_dup=1` (no "Back to signals" button and no `title-3` label in the body); the old
`back_to_list` reading is gone with the button. Help, README and the probe doc follow.

src/ui_app.hpp

@@ -2161,6 +2161,9 @@ gboolean dock_session_probe(gpointer user);  // UDPSCOPE_PROBE_DOCK_SESSION_MS
 /// per setup surface, each page in a scroller of its own.
 void build_slideover(App& app);  // once, after the plot_overlay exists
 bool slideover_add_page(App& app, const std::string& id, const std::string& title, GtkWidget* content);
+/// Retitles a page in place (`slideover_add_page`'s title, for a page whose subject changed: a signal
+/// page carries its channel's number, and the stream can renumber). False when the id is unknown.
+bool slideover_set_title(App& app, const std::string& id, const std::string& title);
 bool slideover_has_page(const App& app, const std::string& id);
 /// The inverse of `slideover_add_page`: false when the id is unknown or is the open page.
 bool slideover_remove_page(App& app, const std::string& id);

src/ui_signal_page.cpp

@@ -194,20 +194,14 @@ void fill(App& app, const std::string& name, GtkWidget* page) {
   const bool enabled = app.model->signal_enabled(index);
   const bool plotted = signal_plotted(app, sig.name);
 
-  // Back to the list: a page reached from a list row has no other way home.
-  GtkWidget* back = text_button("Back to signals", page);
-  gtk_widget_set_halign(back, GTK_ALIGN_START);
-  gtk_widget_add_css_class(back, "flat");
-  g_signal_connect(back, "clicked", G_CALLBACK(+[](GtkButton*, gpointer user) {
-                     slideover_open(*static_cast<App*>(user), "signals");
-                   }),
-                   &app);
-
-  GtkWidget* title = gtk_label_new(sig.name.c_str());
-  gtk_widget_add_css_class(title, "title-3");
-  gtk_widget_set_halign(title, GTK_ALIGN_START);
-  gtk_label_set_ellipsize(GTK_LABEL(title), PANGO_ELLIPSIZE_END);
-  gtk_box_append(GTK_BOX(page), title);
+  // The page's name is the panel header's, not the body's: the slide-over already prints it above the
+  // content (with its **Close** button), so a second copy in the body is the same word twice. The
+  // header's title carries the channel's number -- the same one the badge and the measurements pane
+  // use -- so the panel says which acquired channel this is, not just what it is called.
+  const int channel_number = measure_channel_number(app, sig.name);
+  slideover_set_title(app, page_id(name),
+                      channel_number > 0 ? std::to_string(channel_number) + ". " + sig.name
+                                         : sig.name);
   if (sig.math) {
     if (!sig.unit.empty()) {
       GtkWidget* unit = gtk_label_new(sig.unit.c_str());
@@ -489,8 +483,29 @@ gboolean signal_page_probe(gpointer user) {
               : find_descendant(page_it->second, [](GtkWidget* w) { return is_button_labelled(w, "Remove from plots"); });
     if (off != nullptr) g_signal_emit_by_name(off, "clicked");
   }
+  // The page's *header* names it -- numbered like the badge -- and the body carries no second copy of
+  // the name and no "Back to signals" button: the header's Close is the way out, the badge bar and the
+  // list the ways in. Read before switching to the list.
+  const int channel_number = measure_channel_number(a, name);
+  const std::string want_title =
+      channel_number > 0 ? std::to_string(channel_number) + ". " + name : name;
+  // Copied: the label's text is replaced when the probe switches pages below, and a `const char*` into
+  // it would be read after the free (it printed as an empty title).
+  const std::string shown_title =
+      a.slideover.title != nullptr ? gtk_label_get_text(GTK_LABEL(a.slideover.title)) : "";
+  const int title_ok = a.slideover.page == ("signal:" + name) && want_title == shown_title ? 1 : 0;
+  // ...and the body really has lost the two duplicates: no "Back to signals" button (the header's
+  // Close is the way out) and no second copy of the name (a `title-3` label; the header carries it).
+  GtkWidget* body = page_it == a.signal_pages.end() ? nullptr : page_it->second;
+  const int no_dup =
+      body != nullptr &&
+              find_descendant(body, [](GtkWidget* w) { return is_button_labelled(w, "Back to signals"); }) ==
+                  nullptr &&
+              find_descendant(body, [](GtkWidget* w) { return gtk_widget_has_css_class(w, "title-3"); }) ==
+                  nullptr
+          ? 1
+          : 0;
   slideover_open(a, "signals");
-  const int back_to_list = slideover_page(a) == "signals" ? 1 : 0;
 
   // Activating the signal's row in the list opens its page (the path a click takes). The list proposes what
   // can be added, so the signal has a row only while it is not acquired: switch it off for the leg and back
@@ -660,9 +675,10 @@ gboolean signal_page_probe(gpointer user) {
   std::fprintf(stderr,
                "[signalpage] name=%s open=%d pages=%d enable_visible=%d plot_buttons=%d plot_on_row0=%d "
                "plot_on_row0_disabled=%d "
-               "removed=%d btn_row0=%d back_to_list=%d row_opens=%d pruned=%d other_page_kept=%d rows=%d\n",
+               "removed=%d btn_row0=%d title='%s' title_ok=%d no_dup=%d row_opens=%d pruned=%d "
+               "other_page_kept=%d rows=%d\n",
                name.c_str(), open, pages, enable_visible, plot_buttons, plot_on_row0, plot_on_row0_disabled,
-               removed, btn_row0, back_to_list, row_opens, pruned,
+               removed, btn_row0, shown_title.c_str(), title_ok, no_dup, row_opens, pruned,
                other_page_kept, a.rows->row_count());
   return G_SOURCE_REMOVE;
 }

src/ui_slideover.cpp

@@ -118,6 +118,23 @@ bool slideover_add_page(App& app, const std::string& id, const std::string& titl
   return true;
 }
 
+bool slideover_set_title(App& app, const std::string& id, const std::string& title) {
+  SlideOver& s = app.slideover;
+  if (s.stack == nullptr || title.empty()) return false;
+  GtkWidget* child = gtk_stack_get_child_by_name(GTK_STACK(s.stack), id.c_str());
+  if (child == nullptr) return false;
+  GtkStackPage* page = gtk_stack_get_page(GTK_STACK(s.stack), child);
+  gtk_stack_page_set_title(page, title.c_str());
+  if (is_source_page(id)) {
+    AdwToggle* t = adw_toggle_group_get_toggle_by_name(ADW_TOGGLE_GROUP(s.source_switch), id.c_str());
+    if (t != nullptr) adw_toggle_set_label(t, title.c_str());
+  }
+  // The header shows the *open* page's title, so a retitle of the page on show is visible at once --
+  // a signal page's channel number can change under it when the stream renumbers its channels.
+  if (s.page == id) gtk_label_set_text(GTK_LABEL(s.title), title.c_str());
+  return true;
+}
+
 bool slideover_has_page(const App& app, const std::string& id) {
   return app.slideover.stack != nullptr &&
          gtk_stack_get_child_by_name(GTK_STACK(app.slideover.stack), id.c_str()) != nullptr;