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.

help/C/org.udpscope.app/overview.page

@@ -55,7 +55,20 @@
 
     <p>The signals arrive <em>disabled</em>. Nothing is acquired until you turn one on in the
     signal list, and enabling is what starts acquisition — so a freshly opened scope with an
-    idle side panel is working correctly, not failing to connect.</p>
+    idle side panel is working correctly, not failing to connect. For the same reason a start
+    that has enabled nothing prints no <code>buffer allocated:</code> line on stderr.</p>
+
+    <note style="warning">
+      <p>Over SSH with X11 forwarding the window may never appear, and the cure is
+      <cmd>udpscope --renderer cairo</cmd>. Two symptoms tell the two halves apart: a
+      <code>vkCreateSwapchainKHR() ... VK_ERROR_INITIALIZATION_FAILED</code> warning is
+      GTK's default Vulkan renderer failing to make a swapchain and is what the option's
+      <code>GSK_RENDERER=cairo</code> answers; a window whose allocation stays zero
+      (<code>window_button_w=0</code>) means the plot's GL area has no context either, and
+      that is what its <code>LIBGL_ALWAYS_SOFTWARE=1</code> answers. Both variables are
+      exported before GTK initialises, because they are read when the renderer and the GL
+      context are created. <cmd>--renderer gl</cmd> is faster when indirect GL works.</p>
+    </note>
 
     <note style="tip">
       <p>See <link xref="source">Choosing a source</link> for the chooser and the multicast