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/verification.md

@@ -15,9 +15,11 @@ xvfb-run -a env UDPSCOPE_EXIT_AFTER_MS=8000 UDPSCOPE_ENABLE_ALL_MS=1000 \
 
 Two baselines every run is read against. A clean build emits **four** warnings and no more: two
 `-Wmissing-field-initializers` on `MathNode::source` and `-Woverlength-strings` on **both**
-generated GResource `.c` files — the help pages' and the icons', whose blob crossed C99's
-4095-byte string-literal limit when the two app icons joined the four action ones (a note about a
-generated file, which GCC itself supports). The fifth warning that used to appear here was an unused
+generated GResource `.c` files — the help pages' (**514532** bytes) and the icons', whose blob is
+**8613** against C99's 4095-byte string-literal limit with the five action icons and the two app icons
+that remain (a note about a generated file, which GCC itself supports) -- re-measured when
+`udpscope-math-symbolic` was deleted, which did not move the count; re-measure by touching the two
+`*.gresource.xml` files and rebuilding. The fifth warning that used to appear here was an unused
 `output_built` in `math_probe()`; it was **deleted** as dead code — redundant with the
 `output_after_add` already printed on the same line — and that deletion is what dropped the baseline
 from five to four. Nothing started reading it. And an exit-dump run emits **no** `Gtk-CRITICAL` at
@@ -205,6 +207,12 @@ precedent, not an exception.
   pauses, reports again with `dn=` per measurement, then resets and reports a third time. Three
   phases: `phase=0`, `phase=1` (after pause) and `phase=2` (after Reset). Needs
   `UDPSCOPE_ENABLE_ALL_MS` ahead of it and `UDPSCOPE_PROBE_MEASURE_MS` to add measurements first.
+- `UDPSCOPE_PROBE_SIDE_TAB_MS=3000` with `UDPSCOPE_EXIT_AFTER_MS=11000` — the measurements on the
+  Measure tab, a forced pass, then the tab switched away and read again two seconds later. **`advanced=4`
+  is the assertion** (every accumulator gained passes while its page was off screen); `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. It enables *and plots* its own signal, so it needs no other
+  probe ahead of it. The reading's own entry is in `probe-readings.md`.
 - `UDPSCOPE_PROBE_SEARCH_MS` — runs a buffer search for the armed condition, cross-checks the live
   trigger's own `t0` against the nearest hit (`agree=1` is the load-bearing reading), and jumps to
   hit 0. Reports the resulting `mode=`, `axis=` and `marks placed=`.