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_probe.cpp

@@ -78,6 +78,28 @@ gboolean exit_probe_step3(gpointer user) {
   a->dbg_signal_h = gtk_widget_get_height(a->signal_panel);
   a->dbg_trigger_w = gtk_widget_get_width(a->trigger_panel);
   a->dbg_trigger_h = gtk_widget_get_height(a->trigger_panel);
+  // The switcher's icon as it is *rendered*: a 32x16 glyph in a 32px box draws 1:1, which is what
+  // keeps its 2px bars crisp rather than blurred by a fractional scale. Read here and not on the
+  // first tick, where the window has not been allocated yet and every widget still measures 0x0.
+  if (a->side_switcher != nullptr) {
+    a->dbg_switcher_w = gtk_widget_get_width(a->side_switcher);
+    a->dbg_switcher_h = gtk_widget_get_height(a->side_switcher);
+  }
+  for (GtkWidget* pill = a->side_switcher != nullptr ? gtk_widget_get_first_child(a->side_switcher)
+                                                    : nullptr;
+       pill != nullptr; pill = gtk_widget_get_next_sibling(pill)) {
+    const std::function<void(GtkWidget*)> measure_icon = [&](GtkWidget* node) {
+      if (GTK_IS_IMAGE(node) && gtk_widget_get_mapped(node)) {
+        a->dbg_switcher_icon_w = std::max(a->dbg_switcher_icon_w, gtk_widget_get_width(node));
+        a->dbg_switcher_icon_h = std::max(a->dbg_switcher_icon_h, gtk_widget_get_height(node));
+      }
+      for (GtkWidget* c = gtk_widget_get_first_child(node); c != nullptr;
+           c = gtk_widget_get_next_sibling(c)) {
+        measure_icon(c);
+      }
+    };
+    measure_icon(pill);
+  }
   // The height of one signal row, which is the thing "the channel toggle is too tall"
   // was about. The panel's own height cannot stand in for it: it also covers the
   // heading, the hint and the box margins.