title: Implement a parser from a written specification
check: python -m pytest -q .bench_check
plan: |
  Milestone 1: implement `parse_duration` as `SPEC.md` specifies.

  1. Read all of `SPEC.md`. It has eight numbered rules; every one applies.
  2. In `durations.py`, replace the body of `parse_duration(text: str) -> int`
     so that it follows every rule in `SPEC.md`, including the inputs each
     rule says are invalid and the error message for them. Examples:
     "1h30m" -> 5400; "90" -> 90; "1:02:03" -> 3723.
  3. Create `test_durations.py` with pytest tests: at least one valid and one
     invalid input for each of the eight rules.
  4. Run `python -m pytest -q test_durations.py`, and fix `durations.py`
     until every test passes.
  5. Call finish.