Calculation of available time in ArenaStep wrong

The following was found by Helmut Eller in Emacs. We think the calculation of available time in `ArenaStep` is wong by a factor of `clocks_per_sec` (= 10^6). The attached patch fixes that.

[0001-Fix-bug-calculating-available-time-in-ArenaStep.patch](https://github.com/user-attachments/files/22130979/0001-Fix-bug-calculating-available-time-in-ArenaStep.patch)