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/probe-readings.md

@@ -450,11 +450,25 @@ Two lines of the self-check output are assertions in disguise:
   floor are the two refusals, and because the probe emits `::clicked` straight at the
   widget it reaches the handler even when the button is insensitive, so `add above 0` there
   must leave `rows=` and `traces=` untouched rather than growing a ninth row.
-- `header: window_button_w=W bar_w=B` (stderr, once a second) — **`W` must be
+- `[udpscope] header: window_button_w=W bar_w=B` (stderr, once a second, **only when
+  `UDPSCOPE_PROBE_HEADER` is set**) — **`W` must be
   a small constant and must not track `B`**: the window control is a header-bar child,
   and an expanding one is measured into the title's centre box, so it stretched from the
   label to the title (516 px at a 1270 px bar before the fix, 121 px after). A `W` that
-  grows with the window width is that bug back.
+  grows with the window width is that bug back. The widths are still sampled every second
+  in every run, because the exit dump's `header:` line reads them; only the line is behind
+  the variable, since a reading printed on every run is a log flood (the `[notrace]` rule).
+  An unallocated window reads `0` for both, which is the reading for "the toplevel never
+  got a size", not for a layout drift.
+- `[renderer] option=NAME GSK_RENDERER=V LIBGL_ALWAYS_SOFTWARE=V` (stderr, at startup,
+  before GTK initialises; only when `UDPSCOPE_PROBE_RENDERER` is set) — what `--renderer`
+  exported, printed from `main()` so the reading is of the environment a pre-init GTK will
+  see. `option=(unset)` with both variables `(unset)` is the default and the assertion that
+  the flag changes nothing unless asked.
+- `[renderer] realized=TYPE` (stderr, once, from the first tick that finds a window renderer;
+  same probe) — the class GTK actually built (`GskCairoRenderer` for `--renderer cairo`),
+  which is the half of the claim the export alone cannot make. It is null until the window is
+  realized, so a tick that finds none tries again rather than printing a false reading.
 - `toolbar icons: trigger=NAME(ok) events=... pause=... cursors=... sidebar=... menu=...` (in the
   exit dump, beside `panel controls:`) — the icon name every toolbar button
   carries, with `gtk_icon_theme_has_icon()`'s verdict. **Every entry must read `(ok)`.** Three