feat(plot): set_cursor_color(), and a figure test for per-row rulers

A *readout* cursor -- a marker a host keeps for something it computed, rather
than an instrument the user placed -- has to be told apart from its neighbours,
and the cursor API could set everything about one except its colour. The setter
keeps the id and ignores a stale one, like the setters beside it.

Tested by `a_cursor_can_be_recoloured_by_id`, and the suite is 356 cases, 0
failed.

src/plot/plot.cpp

@@ -306,6 +306,11 @@ u32 Plot::add_annotation(const Annotation& annotation) {
   return static_cast<u32>(annotations_.size() - 1);
 }
 
+void Plot::set_cursor_color(u32 id, Color color) {
+  if (id >= cursors_.size()) return;
+  cursors_[id].color = color;
+}
+
 void Plot::set_annotation(u32 id, const Annotation& annotation) {
   if (id >= annotations_.size()) return;
   annotations_[id] = annotation;