load_benchmark used the JSON filename stem as video_id, which broke
v2-360 questions (all 180 questions got video_id='accepted_questions').
Now uses qa['video_id'] when present, falls back to filename for
Video-MME format compatibility.
- Add skill_target (str | None) and difficulty_steps (int | None) fields
to GeneratedQuestion dataclass with field(default=None)
- Update loader.py to pass new fields from JSON (backward-compatible)
- Update pools.py _q_to_dict/_dict_to_q for serialization compat
- Add question_gen_v2 config section to default.yaml
- Add comprehensive test coverage (7 tests)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>