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.

docs/claude/build-and-run.md

@@ -103,6 +103,12 @@ count to *start* with, 1..`RowSet::kMaxRows`; the layout editor owns it afterwar
 the value is required, and the popover button uses 1000 when the flag was absent).
 The flag is **not** persisted in the session file: persistence of the enable state
 belongs with `todo.md` item 3, alongside the relative-time toggle.
+`--renderer cairo|gl|vulkan` exports `GSK_RENDERER` **at the top of `main()`** —
+before `choose_source()`'s `adw_init()` and `run_ui()`'s, because GSK reads the variable when
+it creates its renderer and setting it later is a silent no-op. `cairo` is the software preset
+and also exports `LIBGL_ALWAYS_SOFTWARE=1`: the plot is a `GtkGLArea`, so a display with no
+usable GPU fails twice, and this is the pair `tools/make-help-shots.sh` already sets. Nothing
+is exported when the option is absent, so GTK's own choice stands.
 
 `tools/mock_udpstreamer.py` has waveform-injection options used by the probes: