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.

help/C/org.udpscope.app/signals.page

@@ -30,7 +30,8 @@
       </item>
       <item>
         <p>Once a signal is acquired its badge appears. Click it to open that signal's page: its
-        details directly under its name — <gui>Unit</gui>, <gui>Gain</gui> and <gui>Offset</gui>,
+        title carries the signal's name and its channel number (<code>2. sine</code>), and its
+        details follow — <gui>Unit</gui>, <gui>Gain</gui> and <gui>Offset</gui>,
         each with a pencil button that edits that one value, then the type, size, range, sample
         rate and timing the stream declared — its colour and <gui>Plot on</gui> buttons for the
         rows.</p>