ui: the tabs are icons, and the pills keep their names

Each sidebar page gets its own glyph and no visible text. The Measure tab reuses the ruler
(`udpscope-measure-symbolic`, the icon its deleted toolbar toggle used and the Plot preferences page
still carries); the Signals tab gets a new `udpscope-signals-symbolic` — three rows of a colour chip
and a name, which is what that tab mostly is — rather than a second copy of the trigger glyph, since
one shape meaning two things in one window is worse than a new one. Its bars are all one length: a
set of unequal ones reads as a bar chart, and a chart is not a list.

`AdwViewSwitcherPolicy.narrow` alone does **not** give an icon-only bar. Measured on libadwaita 1.10
by walking the switcher's tree: with `narrow` and a title, each pill holds a `GtkImage` *and* a
`GtkLabel` — 48px of "Measure" inside a 54px pill. So the pages' `title` is **empty** (their `name` is
what identifies them to `select_side_tab()` and to the session file, and both are unchanged) and the
words are re-attached in code: `name_side_tabs()` runs on the first tick — the pills are
`AdwViewSwitcher`'s own and exist only once it is mapped — and gives each pill a **tooltip and an
accessible name** from the page's name, then hides the empty label that would otherwise leave the
icon off-centre. An icon-only pill is still a named control for a screen reader, and the page icon
that does not resolve leaves a *blank* pill, which is why it is read like the toolbar's icons are.

Two readings, because the claim is two claims:

    switcher: policy=narrow pills=2 labels=0 names=2
    page icons: measure=udpscope-measure-symbolic(ok) signals=udpscope-signals-symbolic(ok)

`labels` counts *mapped labels with text* — a plain mapped count reads 2 either way, because the
narrow pill keeps an empty label in its layout — and `pills=2` is the guard that makes it mean
something: with nothing mapped the rest of the line is void rather than a pass.

Docs: the icon census (six action icons, eight bundled, and what each of the two new users is), the
switcher bullet in `invariants-gtk-and-chrome.md` with the measurement above, the two new dump lines
in `probe-readings.md`, the re-measured GResource figures in `verification.md` (icons 8613 -> 9842
bytes, help 514660 after this session's help rewrite; the count stays four), and the README recording
that the design spec had the switcher showing titles and neither page an icon.

Verified: `make test-ui` 14 files; `[side-tab-probe] … advanced=4` (tab switching by name still
works with empty titles); `signals panel: … divider=1`; `[math-probe] 3 math channels`; zero
`Gtk-CRITICAL` and no missing icon.

docs/claude/invariants-gtk-and-chrome.md

@@ -55,10 +55,14 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   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,events,resize-rows}-symbolic.svg`
-  are the five action icons. `udpscope-math-symbolic` left with the toolbar toggle that was its last
+  measurements and math. `src/icons/scalable/actions/udpscope-{trigger,cursors,measure,events,resize-rows,signals}-symbolic.svg`
+  are the six 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
+  an icon with a live user is not an orphan, whatever the toggle that introduced it did -- and it is
+  *also* the Measure tab's icon. `udpscope-signals-symbolic` was drawn for the Signals tab: three rows of
+  a colour chip and a name, which is what that tab mostly is, and deliberately *not* a second copy of the
+  trigger glyph (one shape with two meanings in one window is worse than a new one). Its bars are all one
+  length because unequal ones read as a bar chart, and a chart is not a list. 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
@@ -67,7 +71,7 @@ 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 seven are bundled by `src/icons/udpscope.gresource.xml` (CMake runs
+  All eight 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,
@@ -195,6 +199,16 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   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.
+- **`AdwViewSwitcherPolicy.narrow` still draws a label; an *empty title* is what makes a pill an icon.**
+  Measured on libadwaita 1.10: with `narrow` and a title, the pill holds a `GtkImage` **and** a
+  `GtkLabel` — 48px of "Measure" inside a 54px pill — so the policy alone does not give an icon-only tab
+  bar. The sidebar's pages therefore have **no title** (their `name` is what identifies them to
+  `select_side_tab()` and the session file) and the pills are *named in code* instead: the first tick
+  calls `name_side_tabs()`, which walks the switcher's children — the pills are there in page order, and
+  `AdwViewSwitcherButton` is not a public type, so there is no type check to make — and sets each pill's
+  **tooltip and accessible name** from the page's name, then hides the empty label that would otherwise
+  leave the icon off-centre. An icon-only pill is still a named control for a screen reader, and
+  `switcher: … pills=2 labels=0 names=2` is the reading for all three facts at once.
 - **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