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.