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-gtk-and-chrome.md

@@ -49,13 +49,17 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   one of them with reality): it is computed on the first *tick*, because the dump
   runs after the window is destroyed and asking GTK about a widget then is four criticals, which is
   how the first version of that line earned them. What stays in the menu is what acts on the *data*
-  (Fit, Export) and the two editors that are not panes (Layout, Signal list). The Arm/Disarm button in the trigger panel still has text.
+  (Fit, Export, Configurations) and the one editor that is not a pane (the layout editor): the signal list
+  is a *tab* of the sidebar, so there is nothing left to reveal or hide about it. The Arm/Disarm button in the trigger panel still has text.
 - **Seven of this app's icons are its own, and they are a `GResource` compiled into the
   binary.** No icon theme has a glyph for an oscilloscope trigger, a pair of cursors or a
   measurement ruler, and the stock names those buttons used to carry said something else
   entirely: a *record dot* for the trigger and three different **magnifiers** for cursors,
-  measurements and math. `src/icons/scalable/actions/udpscope-{trigger,cursors,measure,math,events}-symbolic.svg`
-  are the five action icons; beside them in `src/icons/scalable/apps/` are the two
+  measurements and math. `src/icons/scalable/actions/udpscope-{trigger,cursors,measure,events,resize-rows}-symbolic.svg`
+  are the five action icons. `udpscope-math-symbolic` left with the toolbar toggle that was its last
+  user; `udpscope-measure-symbolic` **stayed**, because the Preferences window's Plot page carries it --
+  an icon with a live user is not an orphan, whatever the toggle that introduced it did. Beside them in
+  `src/icons/scalable/apps/` are the two
   **application** icons, `org.udpscope.app.svg` (128px colour artwork) and
   `org.udpscope.app-symbolic.svg` (16px). The `apps` context is where an icon theme keeps an
   application's own icon and is where the **in-process** window icon resolves from; it is also
@@ -63,7 +67,7 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   into `$XDG_DATA_HOME/icons/hicolor/scalable/apps` and `.../symbolic/apps` — those installed
   copies are what makes `Icon=org.udpscope.app` in the desktop entry resolve (a desktop shell
   cannot read a GResource compiled into the binary).
-  All six are bundled by `src/icons/udpscope.gresource.xml` (CMake runs
+  All seven are bundled by `src/icons/udpscope.gresource.xml` (CMake runs
   `glib-compile-resources`) and reached through **one**
   `gtk_icon_theme_add_resource_path()` in `run_ui()`, right after `adw_init()` — not at the top
   of `build_toolbar()`, where it used to be: there are two consumers now (the window's own icon,
@@ -179,6 +183,18 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   instead of across. `gtk_widget_set_visible()` is what a layout honours, so anything that must not
   take room has to be hidden and not merely unrevealed; the revealer keeps the slide while the pane
   *is* open. The allocated width is how this was found -- the revealer's own measure said 0.
+  **The sidebar is a tab stack now, and has no `GtkRevealer` left in it**: an `AdwViewSwitcher`
+  above an `AdwViewStack`, each page a `GtkScrolledWindow` of its own (the two pages are different
+  heights, so one shared scroller would leave a switch scrolled where the new page never asked to
+  be). The reason is the one this trap teaches: five sections competing for one 320 px column left
+  every one of them short of room. A *feature's* on/off is no longer a visibility at all -- the
+  trigger's and the cursors' toggles set their panel's **sensitivity**, so the panel stays where it
+  is and the settings you are about to switch back on do not vanish while you look for them. The
+  trap itself does not go away with the widget: a stack's hidden page measures 0x0 too and is still
+  given the leftover width, so **ask about state, not about size** carries over unchanged. Every
+  allocation read in the sidebar is a debug capture, and each reads 0 on the page that is *not*
+  current -- a change in what the reading means, not a defect, and no functional code may depend on
+  those numbers.
 - **A list box outlives the children of the pane it sits in, and the teardown order is not yours to
   choose.** Tearing the window down finalizes the events pane's buttons, then the list box, and the
   list's own `selected-rows-changed` then reaches a handler holding the freed pointers: four