feat: the measurements pane can show how each reading varies, and remembers that it was asked to

src/layout_store.hpp

@@ -58,6 +58,11 @@ struct LayoutState {
   /// `math_node_text()` for the field order.
   std::vector<MathChannel> math;
 
+  /// Whether the measurements pane shows its statistics line. The only piece of the
+  /// statistics that is persisted: the accumulated figures themselves describe a session's
+  /// stream and mean nothing restored against another one.
+  bool measure_stats = false;
+
   /// The measurements pane, by signal *name* and with the type and options a row was built with.
   std::vector<Measure> measures;
 };