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.

README.md

@@ -149,7 +149,10 @@ divider's `+` for a new math channel; when every signal is acquired a dim "every
 stands in for the list. While the bar has overflowed into the "N signals" badge the page lists the acquired
 signals too, since their rows are then the only way to each signal's page. There is no signal lock: a
 signal's page and the list always allow acquire, colour, calibration, the math editor and delete.
-A signal badge opens that signal's own page in the slide-over -- the unit, gain and offset and the CONFIG's details directly under the name (aligned rows; a pencil beside the unit, gain and offset edits just that one -- a unit rename is stored per signal name, saved as `[[unit]]` and re-applied to every CONFIG), the colour and the *Plot on N*
+A signal badge opens that signal's own page in the slide-over -- its title is the **numbered name**
+(`2. sine`, the same number the badge and the measurements pane use) and the body carries the unit,
+gain and offset and the CONFIG's details directly under it, with no second copy of the name and no
+"back to the list" button (the header's Close is the way out) (aligned rows; a pencil beside the unit, gain and offset edits just that one -- a unit rename is stored per signal name, saved as `[[unit]]` and re-applied to every CONFIG), the colour and the *Plot on N*
 buttons -- and is also the drag handle: drag it onto a plot to draw the signal there, or onto another
 badge to reorder them. Past **six** signal badges the bar shows a single **"N signals"** badge in their
 place (it opens the list, and `+` is hidden): the E-1608's 8 analog inputs and counter would not fit