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.

索引モデルが issue の本文から書いたものです。