The rotation interval was installed once when the banner mounted and never
touched again, so a swipe, arrow or dot tap inherited whatever was left of the
running countdown — swiping 5.5s into a 6s interval moved the banner on half a
second later.
The timer moves into heroRotation.ts as a small restartable object so it can be
unit-tested, and every manual navigation path restarts it from that moment.
Verified red-first: with restart() reverted to leave a running timer alone, the
regression test fails.
Release 0.9.1.
