ui: the tab glyphs are 32x16 pairs, drawn twice as wide

Each tab's icon is now two motifs side by side instead of one in a 16px square: the Measure pill is the
two cursors beside the ruler, the Signals pill the trigger beside three channel rows. A 16px cell carries
one motif — squeezing a pair into it is what turned both into a smear — and the pill has the width for a
pair in a 300px column with two pills in it.

Two measurements decided the size, both of which I had wrong from the outside:

- **GDK paints an icon into a square box whatever the document's aspect.** `gtk_icon_theme_lookup_icon()`
  at 32 reports a `32x32` paintable for a 32x16 document, and librsvg fits it: rendered with aspect kept,
  the artwork comes out `32x16` inside a 32 request. So the pair is letterboxed, not stretched, and
  `pixel_size` 32 draws it at exactly the size it is written — 1:1, so the 2px bars stay crisp. A
  fractional scale (24 or 48) is what would blur them, which is why nothing here is drawn at one.
- **The bar does not get taller for it.** `switcher: … icon=32x32 size=308x36`: 36px is what the switcher
  measured with 16px icons too, because the pill's fixed height takes a 32px box without growing.

Both glyphs were drawn and then read back as ink maps rather than trusted: the first drafts came out at
39% and 44% ink (a blot beside the house 25–36%), and the tab bar is the last place a shape can be
checked, since no screenshot works under this X setup. The finished two measure 23% and 33%.

Verified: `switcher: policy=narrow pills=2 labels=0 names=2 icon=32x32 size=308x36`;
`page icons: measure=udpscope-tab-measure-symbolic(ok) signals=udpscope-tab-signals-symbolic(ok)`;
`[side-tab-probe] … advanced=4`; the Preferences dialog opened in the same run with no "not present";
`make test-ui` 14 files; zero `Gtk-CRITICAL`. The icons' GResource blob is 11661 bytes, still over the
C99 limit, so the clean-build warning count stays four.

docs/claude/probe-readings.md

@@ -357,13 +357,22 @@ 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
+- `switcher: policy=narrow pills=2 labels=0 names=2 icon=32x32 size=308x36` (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.
+  rest of the line meaningless rather than a pass. `policy=narrow` is read rather than assumed. `icon=`
+  is the box the pills' `GtkImage` is *rendered* in, which proves the 32px override stuck: the tab
+  glyphs are 32x16 documents, and GDK paints an icon into a square box whatever the document's aspect,
+  so the artwork is drawn 32x16 at 1:1 inside that 32x32 box — letterboxed, not stretched, which is what
+  keeps its 2px bars crisp. It reads `0x0` on the first tick, where nothing has been allocated yet, and
+  is therefore read by the exit probe with the rest of the sidebar's geometry. `size=` is the bar's own
+  size, and it is the answer to "what did the wider glyphs cost": `36` is what it was with 16px icons,
+  because the pill's fixed height takes a 32px box without growing — the glyph gets twice the width for
+  no vertical room at all.
 - `page icons: measure=udpscope-tab-measure-symbolic(ok) signals=udpscope-tab-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