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.cpp

@@ -2903,6 +2903,17 @@ void name_side_tabs(App& app) {
       }
     };
     hide_labels(pill);
+    // The tabs' glyphs are 32x16 documents, and the pill has the width for them: drawn at 32 they are
+    // 1:1, so both motifs sit side by side at the size they were authored at and the 2px bars stay
+    // crisp. A 16px box would scale the pair down to a smear, and a fractional scale would blur it.
+    const std::function<void(GtkWidget*)> size_images = [&](GtkWidget* w) {
+      if (GTK_IS_IMAGE(w)) gtk_image_set_pixel_size(GTK_IMAGE(w), 32);
+      for (GtkWidget* c = gtk_widget_get_first_child(w); c != nullptr;
+           c = gtk_widget_get_next_sibling(c)) {
+        size_images(c);
+      }
+    };
+    size_images(pill);
     app.side_tabs_named = true;
   }
 }
@@ -5674,9 +5685,12 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
                 app.dbg_trigger_sensitive, app.dbg_switcher_visible,
                 rows.cursors_visible() ? 1 : 0, rows.time_window(),
                 app.window_limit_s);
-    std::printf("[udpscope]   switcher: policy=%s pills=%d labels=%d names=%d\n",
+    std::printf("[udpscope]   switcher: policy=%s pills=%d labels=%d names=%d icon=%dx%d "
+                "size=%dx%d\n",
                 app.dbg_switcher_policy == 0 ? "narrow" : (app.dbg_switcher_policy == 1 ? "wide" : "?"),
-                app.dbg_switcher_pills, app.dbg_switcher_labels, app.dbg_switcher_names);
+                app.dbg_switcher_pills, app.dbg_switcher_labels, app.dbg_switcher_names,
+                app.dbg_switcher_icon_w, app.dbg_switcher_icon_h, app.dbg_switcher_w,
+                app.dbg_switcher_h);
     std::printf("[udpscope]   mode with trigger on: %s\n", app.dbg_mode_triggered.c_str());
     std::printf("[udpscope]   trigger marker x=%.3f, view x_max=%.3f; model trigger_time=%.3f "
                 "valid=%d\n",