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.

src/help/01-overview.txt

@@ -31,13 +31,14 @@ panel is working correctly, not failing to connect.
 <b>The window</b>
 
 • <b>The header bar</b> -- left to right: the time window (<tt>2 s</tt>), the trigger and the
-  pause, then the pane switches -- signal list, cursors, measurements, math, side panel -- and
-  the menu, which holds Fit to data, the layout editor, Export, Keyboard shortcuts, Help and
-  About.
+  pause, the events, cursors and side-panel switches, and the menu, which holds Fit to data, the
+  layout editor, Export, Keyboard shortcuts, Help and About.
 
-• <b>The side panel</b> -- the trigger, cursors, measurements and math panes at the top, then
-  the signal list: one row per signal with its colour, an enable switch, its name and its unit.
-  Signals are dragged from here onto a plot row.
+• <b>The side panel</b> -- two tabs. <b>Measure</b> holds the cursors readout and the
+  measurements derived from it; <b>Signals</b> holds the trigger and the channel list, where a
+  math channel is a row of that one list under a <tt>MATH</tt> heading whose <tt>+</tt> creates
+  one. One row per channel: its colour, an enable switch, its name and its unit. Channels are
+  dragged from here onto a plot row.
 
 • <b>The plot</b> -- up to 8 rows, each with its own y axis and up to 4 traces, all on one
   shared time axis. Right-click a row for its own menu (fit, add a signal, remove a trace).