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.cpp

@@ -62,6 +62,12 @@ void print_usage(std::FILE* out, const char* argv0) {
                "                       time is then kept for export only\n"
                "  --open FILE          open an exported waveform (.npy) instead of a live source:\n"
                "                       signals from its columns, no socket, no acquisition\n"
+               "  --renderer NAME      render with cairo (software), gl or vulkan. cairo also\n"
+               "                       exports LIBGL_ALWAYS_SOFTWARE=1, because the plot is a\n"
+               "                       GtkGLArea and still needs a GL context. Over SSH/X11\n"
+               "                       forwarding use cairo: Vulkan cannot make a swapchain there\n"
+               "                       and the GL area has no hardware context, so the window\n"
+               "                       never paints or allocates\n"
                "  -h, --help           this text\n",
                argv0);
 }
@@ -113,6 +119,15 @@ Config parse_args(int argc, char** argv) {
       c.capture_packets = static_cast<std::uint32_t>(n);
     } else if (a == "--open" || a.rfind("--open=", 0) == 0) {
       c.open_waveform = value_of(argc, argv, i, "--open");
+    } else if (a == "--renderer" || a.rfind("--renderer=", 0) == 0) {
+      const std::string v = value_of(argc, argv, i, "--renderer");
+      // Only the three a source or a forwarded display actually chooses between. Anything
+      // GTK does not know would fall back with a warning and leave the user reading a
+      // renderer failure that is really a typo, so it is refused here instead.
+      if (v != "cairo" && v != "gl" && v != "vulkan") {
+        fail("--renderer must be cairo, gl or vulkan: " + v);
+      }
+      c.renderer = v;
     } else if (a == "--relative-time") {
       c.relative_time = true;
       c.relative_time_specified = true;