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/main.cpp

@@ -10,6 +10,8 @@
 #include "source_dialog.hpp"
 #include "ui.hpp"
 
+#include <glib.h>
+
 #include <cstdio>
 #include <exception>
 
@@ -26,6 +28,31 @@ int main(int argc, char** argv) {
     return 2;
   }
 
+  // `--renderer` is exported here and nowhere else, because GSK reads `GSK_RENDERER` when it
+  // *creates* its renderer: inside the first `adw_init()`/`gtk_init()`, which `choose_source()`
+  // below may call. Setting it after that point is a silent no-op, so the one place it can be
+  // honoured is before anything touches GTK. Overwrite: a flag on the command line is a
+  // statement about this run, and the environment is only a default it may override.
+  //
+  // `cairo` is the whole software preset and not just GSK's half: the plot is a `GtkGLArea`,
+  // which asks GDK for a GL context of its own whatever GSK draws with. Over X11 forwarding
+  // that request is what leaves the window unallocated (no `buffer allocated:` line, zero
+  // width) once the Vulkan swapchain failure is out of the way, so `LIBGL_ALWAYS_SOFTWARE`
+  // makes Mesa answer it with llvmpipe. This is the pair `tools/make-help-shots.sh` sets.
+  if (!cfg.renderer.empty()) g_setenv("GSK_RENDERER", cfg.renderer.c_str(), TRUE);
+  if (cfg.renderer == "cairo") g_setenv("LIBGL_ALWAYS_SOFTWARE", "1", TRUE);
+  // The startup half of the renderer probe, printed before GTK exists: the request and the
+  // variables this process actually exported. The window's `[renderer] realized=` line is the
+  // other half and says which renderer GTK built.
+  if (g_getenv("UDPSCOPE_PROBE_RENDERER") != nullptr) {
+    const char* gsk = g_getenv("GSK_RENDERER");
+    const char* gl = g_getenv("LIBGL_ALWAYS_SOFTWARE");
+    std::fprintf(stderr, "[renderer] option=%s GSK_RENDERER=%s LIBGL_ALWAYS_SOFTWARE=%s\n",
+                 cfg.renderer.empty() ? "(unset)" : cfg.renderer.c_str(),
+                 gsk != nullptr ? gsk : "(unset)", gl != nullptr ? gl : "(unset)");
+    std::fflush(stderr);
+  }
+
   // No `--server` means the user has not said which source to open, so ask. Before the model:
   // declining must cost no buffer and no socket, and nothing below has to cope with "no source".
   // `--open` is the other answer to that question, and the chooser knows it: its Waveforms page