ztimer assumes staying acquired

[chrysn]

In example 02, ztimer_now is just used in the way it can not be used: its values are compared across durations when no timer is running.

Should we just move that over to a plain counter, or is it the intention to show how time differences are measured? If the latter: do we do it 'raw' / 'we know we don't wrap' by calling ztimer_acquire/release, do we go with the ztimer_remove highly-robust-but-cumbersome way, or do we look into the higher abstractions that might be around?