test: a probe reads the hidden tab's accumulator, and the docs follow the tabs

`UDPSCOPE_PROBE_SIDE_TAB_MS` is the one reading for the design's §5 rule -- the statistics
accumulator keeps running on the hidden page -- which nothing tested before. It switches to
Measure, forces one pass so there is a baseline, switches away to Signals and reads again two
seconds later: `advanced=4` is the assertion, and `advanced=0` is the defect where the
accumulation is gated on the visible tab, which would leave a gap in min/max/avg for exactly as
long as the trigger was on screen.

Two things the probe has to do for itself, and getting either wrong makes `advanced=0` a lie
about the tab:

- it **enables its own signal**, because a signal starts disabled (enabling is what starts
  acquisition) and a probe that waited for one to be on would be reading another probe's side
  effects -- and `UDPSCOPE_ENABLE_ALL_MS` is not part of this run;
- it **assigns that channel to a plot**, because a channel with no trace leaves the live window
  *still*, and a still window is precisely the state `update_measures()` is supposed to skip.
  Measured the hard way: without the assignment the probe read `advanced=0 min_n=0` with the view
  pinned and only four passes in twelve seconds, none of them the tab's doing.

Docs: `probe-readings.md` gains the probe's entry and the signals panel's new shape
(`rows == signals + 1 + math`, `divider=0` a list that lost its `+`), loses the whole `math
panel:` entry, and follows `pane_sensitive=`, the gone `revealed=` field, the six toolbar
controls and the tab-reading `m`/`g` key pairs. `verification.md` gains the probe's bullet and
the re-measured warning figures. The invariants record the sidebar as a switcher above a stack
with a scroller per page, the hidden-page trap that replaces the unrevealed-revealer one for it,
that a feature's on/off is sensitivity rather than visibility, that a math channel is a row of
the signal list, and that a cursors-off readout blanks to `--` with the panel left on screen.
`shortcuts.ui` and the help page's window and sidebar bullets stop describing five revealers.
The README records the deviation from `SPECS.md` that names the panes as separate collapsible
sections.

Verified: `[side-tab-probe] page=signals rows=4 accums=4 advanced=4 min_n=16`;
`signals panel: labels=2 list=1 rows=9 signals=4 math=4 divider=1`; `panels: … (page_current=1)
(sensitive=0) switcher=1`; `toolbar icons:` six controls with every name `(ok)`, with the
Preferences dialog opened in the same run; zero `Gtk-CRITICAL` and no missing icon.

docs/claude/invariants-math-channels.md

@@ -301,6 +301,17 @@ with at most the transient a re-start costs. The exit dump's `math '<name>': fin
 the assertion: `finite` must be about `M` minus the hole share, and `finite=0` beside non-zero
 `hole_slots` is this defect returned.
 
+- **A math channel is a row of the *signal* list, under a `MATH` divider, and there is no math pane.**
+  `append_math_signals_locked()` already puts every channel in `signals()`, so the pane was the same
+  list drawn twice; the divider is what the pane uniquely contributed. It is non-selectable and
+  non-activatable -- a heading that highlighted under the pointer would read as a row you can plot --
+  and it carries the `+` that creates a channel, appended even when there are none, because a `+` that
+  waited for a channel to exist could never create the first one. A math row's menu is *Edit graph…*,
+  *Default colour*, *Delete*: no Information and no Calibrate, because calibration is applied at
+  ingest and a derived channel has no stream to describe, and no rename, because the node editor owns
+  a channel's name and unit and a second place to change them would be a second answer to the same
+  question. A row's tick is the channel's own switch and must reach `App::math` -- the copy the
+  session file is written from -- or the list and the file disagree about what "enabled" was.
 - **A graph *edit* does not move the plan; adding or removing a channel does.** The plan is a partition over
   the channels, so `set_math_channels()` takes a surgical path whenever the *set* is unchanged -- same channels,
   names and enables: the store is never touched, every other channel keeps its samples **and** its runtimes, and