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.