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/invariants-gtk-and-chrome.md

@@ -65,11 +65,17 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   `udpscope-tab-measure-symbolic` is the cursors and the ruler they read against, and
   `udpscope-tab-signals-symbolic` is the trigger's edge above the channel list it acts on. They are not
   reuses because a glyph the toolbar already carries means two things the moment a tab shows it too. Both
-  are 16px composites with four features each, which is about the limit of this size: the Signals list is
-  two rows and not three, so the edge and the list get half the cell each, and the bars in it are all one
-  length because unequal ones read as a bar chart, and a chart is not a list. Their ink fractions (32% and
-  36%) sit inside the range the icons above set, which is the check that a composite has not turned into a
-  blot. Beside them in
+  are **32x16** composites, one motif per half: the Measure pill is the two cursors beside the ruler, the
+  Signals pill the trigger beside the channel list. Wider than tall on purpose — a 16px square cell carries
+  one motif, and squeezing a pair into it is what turns both into a smear — and the pill has the width for
+  it, in a 300px column whose switcher has two pills in it. **GDK paints an icon into a square box whatever
+  the document's aspect** (measured: `gtk_icon_theme_lookup_icon()` at 32 reports `32x32`, and librsvg
+  fits it to `32x16`), so the pair is letterboxed, not stretched — and the pill's image is given
+  `pixel_size` 32 in `name_side_tabs()`, which draws the artwork at exactly the size it is written, so its
+  2px bars stay crisp. A fractional scale is what blurs them, which is why nothing here is drawn at 24 or
+  48. The bars in the Signals list are all one length, because unequal ones read as a bar chart and a chart
+  is not a list; their ink fractions (23% and 33%) sit inside the range the icons above set, which is the
+  check that a composite has not turned into a blot. Beside them in
   `src/icons/scalable/apps/` are the two
   **application** icons, `org.udpscope.app.svg` (128px colour artwork) and
   `org.udpscope.app-symbolic.svg` (16px). The `apps` context is where an icon theme keeps an