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.

docs/claude/invariants-gtk-and-chrome.md

@@ -746,3 +746,14 @@ The `Help` submenu, the contextual help's "?" and the guided tour, and the traps
   each is a `GtkWindow` added to the application, and one left behind keeps the GApplication's
   main loop alive around a window that is gone -- the hang where the exit run never reached its
   dump.
+
+- **A page's name lives in the slide-over header, once.** The header prints the open page's title
+  (from the `GtkStackPage`) beside its **Close** button; a page body that repeats the name is the
+  same word twice, and a body-level "back to the list" button duplicates a navigation the header
+  already owns. Signal pages therefore carry **no** name label and **no** back button
+  (`[signalpage] no_dup=1`), and their title is the **numbered** name -- `<channel number>. <name>`,
+  the same `measure_channel_number()` the badges and the measurements pane use -- so the panel says
+  *which acquired channel* this is rather than only what it is called. `slideover_set_title()`
+  retitles a page in place (the open one included) because that number changes when the stream
+  renumbers its channels; `fill()` sets it on every fill, and a page added by
+  `slideover_add_page()` gets its final title from the `fill()` that follows.

docs/claude/probe-readings.md

@@ -2889,7 +2889,7 @@ left the caller's `false` alone would read `dock_shown=0 default=1 ok=0`. The pr
 ## `[signalpage]`
 
 ```
-[signalpage] name=sine open=1 pages=1 enable_visible=1 plot_buttons=1 plot_on_row0=1 plot_on_row0_disabled=1 removed=1 btn_row0=1 back_to_list=1 row_opens=1 pruned=1 other_page_kept=1 rows=1
+[signalpage] name=sine open=1 pages=1 enable_visible=1 plot_buttons=1 plot_on_row0=1 plot_on_row0_disabled=1 removed=1 btn_row0=1 title='1. sine' title_ok=1 no_dup=1 row_opens=1 pruned=1 other_page_kept=1 rows=1
 ```
 
 `UDPSCOPE_PROBE_SIGNAL_PAGES_MS` builds and drives the page of the first streamed signal. `open=1` (the slide-over shows `signal:<name>`), `pages=1`
@@ -2897,8 +2897,11 @@ left the caller's `false` alone would read `dock_shown=0 default=1 ok=0`. The pr
 `rows=` (one "Plot on" button per figure row), `plot_on_row0=1` (`assign_signal_to_row()` returned true and the signal is plotted, from a state where the probe first took it off every plot), `plot_on_row0_disabled=1`
 (after the page refilled with the signal on plot 1, that plot's "Plot on" button is insensitive, "already on this plot"; `0` is a button that would put the signal in a second slot of
 the same plot), `removed=1`
-(the page's "Remove from plots" took it off), `btn_row0=1` (the "Plot on 1" button, the user's path, put it back), `back_to_list=1` ("Back to signals"
-target), `row_opens=1` (the probe switches the signal off so it is addable, activates its list row and the page opens; the signal is then switched back on), and `pruned=1`: a math channel's page was opened, its **Delete** button clicked, and
+(the page's "Remove from plots" took it off), `btn_row0=1` (the "Plot on 1" button, the user's path, put it back),
+`title=` the slide-over **header's** title while the page is open, `title_ok=1` when it is `<channel number>. <name>` -- the
+same numbering the badge carries (`measure_channel_number`), so a signal page says which acquired channel it is -- and
+`no_dup=1` when the page **body** carries neither a "Back to signals" button (the header's Close is the way out) nor a
+second copy of the name (a `title-3` label); `0` is either duplicate back, `row_opens=1` (the probe switches the signal off so it is addable, activates its list row and the page opens; the signal is then switched back on), and `pruned=1`: a math channel's page was opened, its **Delete** button clicked, and
 the slide-over closed and the page was dropped by `signal_pages_sync()`. `pruned=-1` means no math channel could be added; `0` is a stale page. `other_page_kept=1`: with the `trigger` page open, a signal page that is not on show was pruned (a math channel added, its page built, the
 channel removed) and the slide-over was still on `trigger`, both by `slideover_page()` and by the stack's visible child; `0` is `signal_pages_sync()` parking the stack on the
 signals list while the title and the badge marker still name `trigger`; `-1` when there is no `trigger` page or no math channel could be added.