feat(question_gen): add generate-v2 CLI subcommand and experiment script

- Add generate-v2 subparser with --config, --store-dir, --db-path,
  --seed, and --dry-run arguments to tools/generate_questions.py
- Implement _run_generate_v2 async handler: config loading, video
  discovery, DI client construction, TreeIndex loading, pipeline
  invocation, and result persistence
- Add scripts/generate_questions_v2.sh following build_trees.sh
  conventions (source .env, conda run python path, MODE=mock support)
- Update app/question_gen/__init__.py to export full v2 public API:
  run_pipeline_v2, PipelineConfig, PipelineResult, QuestionFamilySpec,
  ALL_FAMILIES, CandidateQuestion, generate_one_v2, GateReport, run_gates
- Add QuestionGenStore.load_progress() for pipeline resumption
- Add integration tests for CLI help and dry-run behavior
- Update test_question_gen_api to match expanded __all__

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/generate_questions_v2.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+# v2 出题管线实验脚本
+# 职责：调用 generate-v2 子命令，基于家族特化 + 四门质量检查生成新题目。
+#
+# 用法:
+#   bash scripts/generate_questions_v2.sh                           # 全量生成
+#   MODE=mock bash scripts/generate_questions_v2.sh                 # smoke test（mock LLM/VLM）
+#   SEED=123 bash scripts/generate_questions_v2.sh                  # 指定种子
+#   CONFIG=config/custom.yaml bash scripts/generate_questions_v2.sh # 自定义配置
+#
+# 环境变量:
+#   STORE_DIR   — store 根目录（默认 store）
+#   CONFIG      — 管线配置 YAML（默认 config/default.yaml）
+#   DB_PATH     — QuestionGenStore SQLite 路径（默认 logs/question_gen.db）
+#   SEED        — 随机种子（可选，覆盖 config 中的 seed）
+#   MODE        — 设为 "mock" 启用 LLM/VLM mock 模式
+#
+# 日志输出:
+#   stderr → 终端实时显示
+#   logs/generate_questions.log → 完整日志
+#   logs/generate_v2_telemetry.db → LLM/VLM 调用遥测
+#   logs/question_gen.db → 出题管线运行日志
+
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+
+STORE_DIR="${STORE_DIR:-store}"
+CONFIG="${CONFIG:-config/default.yaml}"
+DB_PATH="${DB_PATH:-logs/question_gen.db}"
+
+export PYTHONUNBUFFERED=1
+
+# shellcheck source=../.env
+source .env
+
+[ "${MODE:-}" = "mock" ] && export LLM_MOCK=1 VLM_MOCK=1
+
+PYTHON="$(conda run -n Video-Tree-TRM which python)"
+
+"${PYTHON}" tools/generate_questions.py generate-v2 \
+    --store-dir "$STORE_DIR" \
+    --config "$CONFIG" \
+    --db-path "$DB_PATH" \
+    ${SEED:+--seed "$SEED"}