Incorrect definition generated in a case when work has no start, but the next same-level entry has a time

This genereates an invalid definition (`-13:00`), instead of raising an error:

```
* 10:00. blah
- work
* 11:30. blah
- blah
- blah
* 12:30. blah
-----
- 13:00. blah
```

Look at the schedule parsing logic, likely in a file like lib/schedule_manager/parser.rb. The issue shows a schedule snippet where a work entry without a start time precedes a same-level entry with a time, causing an invalid definition '-13:00'. Find where durations or definitions are computed and add validation to raise an error instead. Run existing tests to ensure the fix doesn't break valid cases.

Vom Indexierungsmodell aus dem Issue-Text verfasst.