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/ui/main-window.ui

@@ -52,14 +52,25 @@
                heights, so one shared scroller around the stack would leave a tab switch scrolled
                to somewhere the new page never asked for. The switcher is the sibling above, not a
                child of either scroller: the one control that chooses what you are looking at must
-               not be the first thing to scroll away. -->
+               not be the first thing to scroll away.
+
+               The policy is `narrow` and the **titles are empty**: `narrow` still draws a label
+               beside an icon (measured: a 48px "Measure" inside a 54px pill), so an empty title is
+               what makes a pill an icon. The pages' *names* are what identify them to
+               `select_side_tab()` and the session file, and the words are re-attached to the pills in
+               code — tooltip and accessible name — by `name_side_tabs()` on the first tick, so an
+               icon-only pill is still a named thing rather than an anonymous button.
+               Each page's icon is its own glyph: `udpscope-measure-symbolic` is the ruler the
+               measurements used as a toggle, and `udpscope-signals-symbolic` is the channel list,
+               drawn for this tab rather than reusing the toolbar's trigger icon — one glyph
+               meaning two things in one window is worse than a new one. -->
           <object class="GtkBox" id="side_column">
             <property name="orientation">vertical</property>
             <property name="width-request">300</property>
             <child>
               <object class="AdwViewSwitcher" id="side_switcher">
                 <property name="stack">side_stack</property>
-                <property name="policy">wide</property>
+                <property name="policy">narrow</property>
                 <property name="margin-top">4</property>
                 <property name="margin-bottom">4</property>
                 <property name="margin-start">6</property>
@@ -72,7 +83,9 @@
                 <child>
                   <object class="AdwViewStackPage">
                     <property name="name">measure</property>
-                    <property name="title" translatable="yes">Measure</property>
+                    <property name="title"></property>
+                    <!-- The ruler: what the pane measures the waveform with. -->
+                    <property name="icon-name">udpscope-measure-symbolic</property>
                     <property name="child">
                       <object class="GtkScrolledWindow">
                         <property name="hscrollbar-policy">never</property>
@@ -89,7 +102,9 @@
                 <child>
                   <object class="AdwViewStackPage">
                     <property name="name">signals</property>
-                    <property name="title" translatable="yes">Signals</property>
+                    <property name="title"></property>
+                    <!-- The channel list this page mostly is. -->
+                    <property name="icon-name">udpscope-signals-symbolic</property>
                     <property name="child">
                       <object class="GtkScrolledWindow">
                         <property name="hscrollbar-policy">never</property>