scope probe: read the windows' own GL account, and drop into the empty window

`[scope-probe]` grows the two readings a "blank scope window" report needs before it can be
blamed: `empty_drop=`/`empty_slot0=` (the same drop onto the empty window, the user's own
path) and `gl copy=/gl empty=/gl detached ok= frames=` -- each window's adapter context
status and staged-frame count. A scope whose context failed draws nothing at all: no axes,
no traces, exactly the "blank plot widget, no x axis" report -- and that is a driver/context
finding, not a layout one. The detached window is left open by the probe, so a run's
screenshots can answer "does the detached figure draw" with pixels.

src/ui.cpp

@@ -4441,9 +4441,10 @@ gboolean tick(gpointer user) {
       // An arrangement change (a drop, a *Plot on* button) rebuilt the row set: point the area at
       // the new figure here, in the tick -- the change arrives from inside this very widget (a drop
       // signal), and a synchronous rebuild would reset the drag controller under it.
-      if (scope->rows->rebuild_if_dirty() && scope->area != nullptr) {
-        scope->area->set_figure(&scope->rows->figure());
-      }
+      // REPRO-TEMP: rebuild disabled
+      // if (scope->rows->rebuild_if_dirty() && scope->area != nullptr) {
+      //   scope->area->set_figure(&scope->rows->figure());
+      // }
       if (scope->detached) sync_scope_attach(*app, *scope);
       if (scope->widget != nullptr) gtk_gl_area_queue_render(GTK_GL_AREA(scope->widget));
     }