udpscope: an oscilloscope view for a UDPStreamer source

GTK4 and libadwaita. It receives one UDP stream through the vendored
udpsclient, buffers it in a large ring, and draws it with the vendored
plotter: a trigger with a pre/post event window, a pause that freezes the
window, cursors, measurements, math channels in a node editor, an event cache,
named configurations, preferences, and an export of the visible window to
.npy/.csv. Waveform files can be opened and analysed in place.

The interface is two threads -- a producer that owns the socket and the GTK
main thread that owns every widget and the GL context -- sharing one model
under one mutex, with samples crossing through lock-free rings.

deps/plotter is a submodule pinned to the commit this tree builds against;
deps/udpsclient is vendored in place. docs/claude/ documents the rules that
cost a defect to learn, and SPECS.md is the requirement document.

src/ui_widgets.cpp

@@ -118,6 +118,21 @@ GtkWidget* menu_separator() {
   return gtk_separator_new(GTK_ORIENTATION_HORIZONTAL);
 }
 
+/// A labelled row whose control keeps its *natural* width. A `GtkSwitch`'s natural width is its size,
+/// so `labeled_box()`'s expanding child stretches it into a wide pill that reads as a slider -- this is
+/// the row a switch wants, with the label taking the slack instead.
+GtkWidget* labeled_switch_row(const char* label, GtkWidget* control) {
+  GtkWidget* row = gtk_box_new(GTK_ORIENTATION_HORIZONTAL, 6);
+  GtkWidget* text = gtk_label_new(label);
+  gtk_widget_set_halign(text, GTK_ALIGN_START);
+  gtk_widget_set_hexpand(text, TRUE);
+  gtk_box_append(GTK_BOX(row), text);
+  gtk_widget_set_halign(control, GTK_ALIGN_END);
+  gtk_widget_set_valign(control, GTK_ALIGN_CENTER);
+  gtk_box_append(GTK_BOX(row), control);
+  return row;
+}
+
 GtkWidget* labeled_box(const char* label, GtkWidget* child) {
   GtkWidget* box = gtk_box_new(GTK_ORIENTATION_HORIZONTAL, 6);
   GtkWidget* l = gtk_label_new(label);