fix: the guided tour selects the page a stop's target lives on

A stop whose target is on another page of a stack was entered without
switching to that page, so the spotlight aimed at a widget that was not
on screen. A stop now declares the page it stands on (`TourStop::page`)
and the tour selects it as the step is entered -- the window's trigger
and signals stops put the sidebar on Signals, the chooser's tabs stop
shows the Waveforms page and stepping off it brings Sources back.

The stop filter moved into `tour_begin()` and now runs on
`gtk_widget_get_mapped()`: GTK 4.24's `is_visible()` ignores
`child_visible`, so it kept a stop on another page *and* one in a
collapsed pane alike. A wrong page is kept and selected; a collapsed
pane, whose stack is unmapped too, is still dropped.

docs/claude/verification.md

@@ -323,6 +323,14 @@ as distinct from the fault injection below, which exercises ingest:
   bullet) — so a probe run against a fresh config directory will have it on screen for the first
   second and a half, and that start *writes* `tour_seen` to whichever config directory the run is
   using, which is why the second run against the same one reads `auto=0`.
+- `UDPSCOPE_PROBE_TOUR_SURFACE_MS` — the tour's *surface* switch: it puts the sidebar on **Measure**,
+  starts the window's tour, and walks it a stop per tick to the signals stop, reading the sidebar's
+  page and the spotlight/card where the walk arrives. It then puts the sidebar *away* and starts the
+  tour again, which is the same filter's other case (a collapsed pane drops the stops a page switch
+  could have revealed). `[toursurface] signals stop=4/5 … side_tab=signals … contains=1` and
+  `[toursurface] collapsed: side_stack_mapped=0 steps=2 signals_stop=0` are the two assertions; its
+  lines are in [probe-readings.md](probe-readings.md). Run it alone with
+  `UDPSCOPE_EXIT_AFTER_MS=9000 UDPSCOPE_EXIT_DUMP=1` against a live source.
 - `UDPSCOPE_PROBE_DEFAULTS_MS` — opens **Calibrate** and reads every spin button in it and in the
   trigger panel: `[defaults] calibrate: 1 0 model=1 0 | trigger: 0.001 0.002 0 0.001 0 50 0.01`. A
   `GtkSpinButton` starts at its range's *minimum*, so a `-1e12` here is an unseeded button the
@@ -341,10 +349,16 @@ as distinct from the fault injection below, which exercises ingest:
   on, `hints=[0 {}] hint=0x0` with it off. `UDPSCOPE_PROBE_CHOOSER_HELP=1` drives the **Enable
   contextual help** row and reads the file back; `UDPSCOPE_PROBE_CHOOSER_TOUR=1` drives the **Guided
   tour** row and walks the chooser's own tour to its last stop, printing `[sources] chooser tour: …`
-  and the handoff (`tour_pending=1`); `UDPSCOPE_PROBE_CHOOSER_TOUR_MS=<ms>` schedules the same walk,
-  which is how the **first-run** auto-start is read (schedule it after 1500 ms, and pair it with
-  `UDPSCOPE_PROBE_PICK_MS` a little later so the application's half follows). Its lines and what each
-  asserts are in [probe-readings.md](probe-readings.md). A legacy file
+  and the handoff (`tour_pending=1`); `UDPSCOPE_PROBE_CHOOSER_TOUR_MS=<ms>` walks it a stop per tick
+  instead, printing the page the stack shows at every stop and stepping **Back** off the stop that
+  stands on the waveforms page — the reading for the surface a stop changes. It starts the walk
+  itself when one is not running, so it does not need a first-run config; pair it with
+  `UDPSCOPE_PROBE_PICK_MS` after the walk so the application's half follows. A working run: seed a
+  source once (`./build/udpscope --server 127.0.0.1 --port N` against a mock, under the throwaway
+  `XDG_CONFIG_HOME`), then run bare with
+  `UDPSCOPE_PROBE_CHOOSER_TOUR_MS=2000 UDPSCOPE_PROBE_PICK_MS=4500 UDPSCOPE_EXIT_AFTER_MS=12000
+  UDPSCOPE_EXIT_DUMP=1`. Its lines and what each asserts are in
+  [probe-readings.md](probe-readings.md). A legacy file
   (`<host>_<port>.toml`) produces `layout loaded from …/<host>_<port>/default.toml`, an `active`
   holding `default`, and the old file renamed `.imported`.
 - **The loading window covers the gap, and only on the path that asked.** The same