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.

include/plot/plot/plot.hpp

@@ -479,6 +479,11 @@ class Plot {
   ///
   /// Ignored for a vertical cursor, whose snap is `SnapMode` -- see `set_cursor_snap()`.
   void set_cursor_h_snap(u32 id, HRulerSnap mode);
+  /// Recolours a cursor that already exists. A *readout* cursor -- one a host keeps as a
+  /// marker for something it computed, rather than an instrument the user placed -- has to
+  /// be told apart from its neighbours, and its colour is how it says which readout it
+  /// belongs to. An id that names no cursor is ignored, like every other setter here.
+  void set_cursor_color(u32 id, Color color);
 
   /// The measurement for `id`. Re-evaluated only when a cursor moved, when the
   /// measure was added, or when the series underneath it changed: a different