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-readouts-and-cursors.md

@@ -54,6 +54,12 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   unit and a spaced `m` alone reads as metres; with one it stays spaced and the prefix joins the
   unit (`636.62 mV`). Below `1e-12` the ladder runs out, and `%.2f` would print `0.00` again, so
   that range falls back to `%.3g`.
+- **With the instruments off the readout blanks, and the panel stays on screen.** Cursors-off makes the
+  pane *insensitive* rather than hiding it -- a panel you can still see is a panel you can read, and the
+  settings you are about to switch back on must not vanish while you look for them -- and every cell in
+  it is written `"--"`, the spelling an unavailable cell already uses. A readout quotes a stored sample;
+  with nothing bracketing one there is no sample to quote, and keeping the last values would be quoting
+  a sample the cursors no longer name. Two spellings of one state in one grid would be the defect.
 - **The cursors are read back from the plots only while a gesture is on that row.** `Cursor::x`
   is the *free* pointer position while a drag runs and the snapped value otherwise -- which is
   what the app itself wrote a frame earlier, or the snap of it. A tick that reads a row on