Add --renderer for displays with no usable GPU; gate the header reading

GTK4 defaults to Vulkan, and an X11-forwarded display has no device to make a
swapchain from; the plot is a GtkGLArea, so it then asks for a GL context that
is not there either. --renderer cairo is the software preset for that display:
it exports GSK_RENDERER=cairo and LIBGL_ALWAYS_SOFTWARE=1 in main() before
gtk_init()/adw_init(), because both are read when the renderer and the context
are created. --renderer gl is the faster choice when indirect GL works, and
--renderer vulkan makes the default explicit. Nothing is exported, and GTK's
own choice stands, when the option is absent.

The once-a-second header line is now behind UDPSCOPE_PROBE_HEADER, following
the [notrace] rule: its widths are still sampled for the exit dump, but the
line is detail behind that reading rather than output every run prints.
UDPSCOPE_PROBE_RENDERER prints the exported pair before GTK starts and the
GskRenderer GTK actually built.

README, the manual's overview and docs/claude record the two symptoms and the
cure.

src/config.hpp

@@ -60,6 +60,15 @@ struct Config {
   /// and the chooser's Waveforms page sets this rather than the connection fields.
   std::string open_waveform;
 
+  /// `--renderer NAME`: the GSK renderer to force, by exporting `GSK_RENDERER` **before any GTK
+  /// call**. Empty leaves GTK's own choice alone. `cairo` is the software preset over SSH/X11
+  /// forwarding: it also exports `LIBGL_ALWAYS_SOFTWARE=1`, because the plot's `GtkGLArea` asks
+  /// for a GL context whatever GSK draws with. `gl` is the faster alternative when indirect GL
+  /// works. The variables are read when the renderer/context is created, so setting them anywhere
+  /// after `gtk_init()`/`adw_init()` is a no-op -- which is why this is a `Config` field applied
+  /// at the top of `main()` and not a preference.
+  std::string renderer;
+
   /// The source chooser's "Guided tour" entry. The tour is drawn on the application's own window,
   /// which does not exist while the chooser runs, so the request rides on the one value `main()`
   /// owns and hands to both the chooser and the front end (`open_waveform` and the preferences'