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/probe-readings.md

@@ -357,6 +357,18 @@ Two lines of the self-check output are assertions in disguise:
   substitutes a missing-image placeholder, the button still works, and every other line of the
   dump still passes. Changing an icon name is a one-line change in `build_toolbar()` and this
   is the only check that it names something that exists.
+- `switcher: policy=narrow pills=2 labels=0 names=2` (in the exit dump, beside `panels:`) — the two
+  sidebar tabs as the switcher actually draws them. **`pills=2 labels=0`** is the claim: two pills are on
+  screen and **no text is drawn** (`labels` counts *mapped labels with text*, because `narrow` keeps an
+  empty label in the pill's layout and a plain mapped count would read 2 either way). `names=2` is the
+  other half — both pills carry a tooltip and an accessible name set by `name_side_tabs()`, so an
+  icon-only pill is not an anonymous button — and `pills=0` means nothing is mapped, which makes the
+  rest of the line meaningless rather than a pass. `policy=narrow` is read rather than assumed.
+- `page icons: measure=udpscope-measure-symbolic(ok) signals=udpscope-signals-symbolic(ok)` (in the exit
+  dump, beside `toolbar icons:`) — each tab's own glyph and `gtk_icon_theme_has_icon()`'s verdict, read
+  on the first tick for the same reason the toolbar's are. **Both must read `(ok)`**: the page titles are
+  empty on purpose, so an icon that does not resolve leaves a *blank* pill, and a blank pill looks
+  exactly like a page with nothing in it.
 - `app icons: org.udpscope.app(ok 128x128) org.udpscope.app-symbolic(ok 16x16)` (in the exit dump,
   beside `toolbar icons:`) — the application icon, named and present at the size its own root
   `<svg>` element declares. **Both entries must read `ok` at those sizes.** `ok` is