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/verification.md

@@ -15,11 +15,12 @@ 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' (**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
+generated GResource `.c` files — the help pages' (**514660** bytes) and the icons', whose blob is
+**9842** against C99's 4095-byte string-literal limit with the six 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
+the tab work changed the set (that icon left, `udpscope-signals-symbolic` arrived, and this file's
+help page was rewritten), which did not move the count; re-measure by touching each `*.gresource.xml`
+under `src/` 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