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);