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.

src/icons/scalable/actions/udpscope-signals-symbolic.svg

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- SPDX-License-Identifier: MIT -->
+<!-- SPDX-FileCopyrightText: 2026 Martino Ferrari -->
+<!-- The Signals tab's own glyph: the channel list, drawn as a list. Three rows of a
+     colour chip and a name, the shape the signal panel's own rows have — which is
+     what the tab mostly *is*, and which is why it is not the trigger icon: the
+     toolbar's trigger button already carries that one, and one glyph with two
+     meanings in one window is worse than a new glyph. The bars are all one length
+     on purpose: unequal ones read as a bar chart, and a chart is not a list. Every
+     coordinate is a whole pixel of the 16px cell, every subpath is a filled `path`
+     wound the same way, and the 2px bars match the house stroke weight. -->
+<svg height="16px" width="16px" viewBox="0 0 16 16" xmlns="http://www.w3.org/2000/svg">
+  <path fill="#2e3436" d="M1 1 H4 V4 H1 Z
+                          M6 2 H14 V4 H6 Z
+                          M1 6 H4 V9 H1 Z
+                          M6 7 H14 V9 H6 Z
+                          M1 11 H4 V14 H1 Z
+                          M6 12 H14 V14 H6 Z"/>
+</svg>