Struggling to put this in words that make sense, but the gist of what happened is:

• schedule.ValidUntil() returns its end time
• that end time can match (exactly) the end time of the active window
• whether a time is considered "in" a window is based on inclusive start and end times
• we detect that the end time of the window is "in" the window, even though there is zero time left in the window
• since there is zero time left, we make no progress that iteration (schedule.total() == 0)
• but since schedule.ValidUntil() returns the same end time, we do not make it past the window that we are at the end at.

To solve this, I:

1. made the end time of a window exclusive so we can not end up in a situation where we make zero progress and also do not progress the interval
2. because this is a complex loop condition, I also added a failsafe which returns early if we make no progress

This previously looped forever. Now it does not loop forever, and the non-nil check validates that we are not hitting the failsafe

Becuase I was still uncomfortable with this loop, I added a fuzz test which I ran for ~10 minutes. It found no issues, but was able to surface the issues this PR fixes.