Header-navigation policy for <cngx-stepper>. 'none' renders the
step headers as inert labels - the footer is the sole navigation
control. 'visited' (default) keeps the headers as focusable
buttons; reachability folds into the linear axis (linear=false
is free click-through, linear=true is backward-to-visited only,
with linear-unreachable headers marked aria-disabled). There is no
discrete 'free' value - 'free' is 'visited' + linear=false.