feat(types): extend GeneratedQuestion with skill_target & difficulty_steps

- 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>

config/default.yaml

@@ -69,3 +69,25 @@ harness:
   eval_min_per_class: 2
   early_stop_patience: 8
   use_slow_momentum: true
+
+# ── 出题管线 v2 ──
+question_gen_v2:
+  family_ratios:
+    retrieval: 0.30
+    reasoning: 0.25
+    enumeration: 0.20
+    visual: 0.15
+    spatial: 0.10
+  gate:
+    blind_answer_model: "gpt-4.1-mini"
+    leak_test_model: "gpt-4.1-mini"
+    key_verify_model: "gpt-4.1-mini"
+    multi_true_model: "gpt-4.1-mini"
+  dedup_threshold: 0.85
+  retry_limit: 3
+  heavy_sample_rate: 0.15
+  heavy_agent_model: "gpt-4.1-mini"
+  output_dir: "store/questions/generated-v2"
+  per_type: 20            # 12 类 x 20 = 240 题（设计 §3 硬约束）
+  concurrency: 4
+  seed: 42