ui: the signals panel's frame is a .ui file

src/ui/signals-panel.ui holds the panel's shape: the box and its margins, the heading,
the hint that says what a signal is for, and the list. The rows stay in code -- one per
signal, carrying that signal's switch, colour, calibration and drop targets, rebuilt
whenever the stream's signals change -- so the file holds the frame, which is the shape
the plan gives this slice.

The list's width-only size request moved with it, comment and all: a height floor there
made the list occupy 120 px whatever it held, which hid the compact rows' height behind
it.

Read back by a new dump line, `signals panel: labels=2 list=1`, where a lost id reads as
`labels=1` or `list=0` instead of a panel quietly missing its hint; `panels: … row_h=26`
still reads the list's geometry and the height a row takes. check_ui is at 10 ok.

src/ui.cpp

@@ -5409,6 +5409,20 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
                 static_cast<unsigned long long>(model.trigger_latches().load()),
                 static_cast<unsigned long long>(app.dbg_latches_at_disarm));
     std::printf("[udpscope]   window control: '%s'\n", app.dbg_window_text.c_str());
+    {
+      // The signals panel's frame, read from the window rather than from the `.ui`: the two labels and the
+      // list the file gives it. A `.ui` that lost one of them reads here -- `labels=1` or `list=0` -- rather
+      // than showing as a panel quietly missing its hint.
+      GtkWidget* panel = app.signal_list != nullptr ? gtk_widget_get_parent(app.signal_list) : nullptr;
+      int labels = 0;
+      int lists = 0;
+      for (GtkWidget* c = panel != nullptr ? gtk_widget_get_first_child(panel) : nullptr; c != nullptr;
+           c = gtk_widget_get_next_sibling(c)) {
+        if (GTK_IS_LABEL(c)) ++labels;
+        if (GTK_IS_LIST_BOX(c)) ++lists;
+      }
+      std::printf("[udpscope]   signals panel: labels=%d list=%d\n", labels, lists);
+    }
     std::printf("[udpscope]   panels: side=%dx%d (visible=%d) signals=%dx%d row_h=%d "
                 "(revealed=%d) trigger=%dx%d (revealed=%d) toggles=%d cursors=%d window=%.3fs "
                 "window_cap=%.1fs\n",