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.

src/ui_app.hpp

@@ -219,6 +219,14 @@ struct App {
   int dbg_switcher_pills = -1;
   int dbg_switcher_labels = -1;
   int dbg_switcher_names = -1;
+  /// The pills' icon as it is *rendered*: a 32x16 glyph in a 32px box is drawn 1:1, which is what
+  /// keeps its 2px bars crisp.
+  int dbg_switcher_icon_w = -1;
+  int dbg_switcher_icon_h = -1;
+  /// The switcher's own size, beside it: the pills carry 32px icon boxes now, so the tab bar's height
+  /// is what it costs.
+  int dbg_switcher_w = -1;
+  int dbg_switcher_h = -1;
   int dbg_switcher_policy = -1;
   /// Whether `name_side_tabs()` has named the pills: it can only run once the switcher is mapped.
   bool side_tabs_named = false;