feat: wire grounded selector into AR slot processing

将 Task 5 的 grounded selector 织入 AR 出题路径（Phase 3.5，位于
record_item 与 postprocess 之间），仅在 strategy.uses_grounded_selector
为真时进入。observation 始终落库（含 hard-fail），硬失败走重出。
PipelineConfig 新增 candidate_pool_size/selector_delta_low/
selector_delta_high 三参，YAML 与 CLI seed override 同步。

app/question_gen/pipeline_v2.py

@@ -24,6 +24,7 @@
 from __future__ import annotations
 
 import asyncio
+import json
 import random
 import uuid
 from dataclasses import dataclass
@@ -98,6 +99,9 @@ class PipelineConfig:
         concurrency: 并发 slot 数上限。
         seed: 随机种子。
         output_dir: 输出目录。
+        candidate_pool_size: grounded selector 首轮候选干扰项数 N。
+        selector_delta_low: 干扰项视觉分与正解的最小差（区间上界）。
+        selector_delta_high: 干扰项视觉分与正解的最大差（区间下界）。
     """
 
     per_type: int
@@ -107,6 +111,9 @@ class PipelineConfig:
     concurrency: int
     seed: int
     output_dir: Path
+    candidate_pool_size: int = 24
+    selector_delta_low: float = 0.05
+    selector_delta_high: float = 0.35
 
 
 # ---------------------------------------------------------------------------
@@ -150,6 +157,9 @@ def load_pipeline_config(yaml_path: Path) -> PipelineConfig:
         concurrency=int(section["concurrency"]),
         seed=int(section["seed"]),
         output_dir=Path(section["output_dir"]),
+        candidate_pool_size=int(section.get("candidate_pool_size", 24)),
+        selector_delta_low=float(section.get("selector_delta_low", 0.05)),
+        selector_delta_high=float(section.get("selector_delta_high", 0.35)),
     )
 
 
@@ -281,6 +291,56 @@ def _to_generated_question(
     )
 
 
+def _extract_correct_text(options: tuple[str, ...], answer: str) -> str:
+    """从四选项中取正解文本（去掉 "X. " 字母前缀）。
+
+    参数:
+        options: 选项元组，格式 ("A. ...", "B. ...", ...)。
+        answer: 正解字母（大小写不敏感）。
+
+    返回:
+        正解选项去前缀后的文本。
+
+    异常:
+        ValueError: answer 对应索引超出选项范围。
+    """
+    idx = ord(answer.strip().upper()) - ord("A")
+    if not 0 <= idx < len(options):
+        msg = f"answer '{answer}' 超出选项范围 (n={len(options)})"
+        raise ValueError(msg)
+    opt = options[idx]
+    prefix = f"{answer.strip().upper()}. "
+    return opt[len(prefix) :] if opt.startswith(prefix) else opt
+
+
+def _replace_candidate_options(
+    candidate: CandidateQuestion, options: tuple[str, ...], answer: str
+) -> CandidateQuestion:
+    """用 selector 重组的选项/答案替换候选（CandidateQuestion frozen）。
+
+    参数:
+        candidate: 原候选题目。
+        options: grounded selector 重组后的四选项。
+        answer: 重组后的正解字母（恒 "A"）。
+
+    返回:
+        仅替换 options/answer、其余字段照搬的新 CandidateQuestion。
+    """
+    return CandidateQuestion(
+        question_id=candidate.question_id,
+        video_id=candidate.video_id,
+        task_type=candidate.task_type,
+        skill_target=candidate.skill_target,
+        question=candidate.question,
+        options=options,
+        answer=answer,
+        source_nodes=candidate.source_nodes,
+        difficulty=candidate.difficulty,
+        subtitle_sentences=candidate.subtitle_sentences,
+        frame_paths=candidate.frame_paths,
+    )
+
+
 # ---------------------------------------------------------------------------
 # 单 Slot 处理（重出循环）
 # ---------------------------------------------------------------------------
@@ -428,6 +488,51 @@ async def _process_one_slot(
                 sub_pattern=sub_pattern.name if sub_pattern else None,
             )
 
+            # Phase 3.5: grounded selector（仅 AR 路径）
+            if strategy.uses_grounded_selector:
+                from app.question_gen.distractor_selector import (
+                    SelectorConfig,
+                    build_grounded_options,
+                )
+
+                correct_text = _extract_correct_text(candidate.options, candidate.answer)
+                selector_cfg = SelectorConfig(
+                    candidate_pool_size=config.candidate_pool_size,
+                    delta_low=config.selector_delta_low,
+                    delta_high=config.selector_delta_high,
+                )
+                try:
+                    outcome = await build_grounded_options(
+                        vlm=vlm,
+                        question=candidate.question,
+                        correct_text=correct_text,
+                        material=material,
+                        config=selector_cfg,
+                        session_id=session_id,
+                    )
+                except (ValueError, FileNotFoundError) as e:
+                    logger.warning(
+                        "slot {} selector 异常 (attempt {}): {}", slot.slot_id, attempt, e
+                    )
+                    prev_reason = f"selector_error: {e}"
+                    continue
+
+                # observation 始终落库（含 hard-fail），供 EOB 退化观测与调参
+                store.update_selector_scores(
+                    item_id, json.dumps(outcome.observation, ensure_ascii=False)
+                )
+
+                if outcome.hard_fail:
+                    prev_reason = "grounded 干扰项不足（selector 硬失败）"
+                    store.mark_item_rejected(item_id, prev_reason)
+                    logger.info("slot {} selector 硬失败 (attempt {})", slot.slot_id, attempt)
+                    continue
+
+                # 用 grounded 四选项替换候选（frozen → 构造新实例）
+                candidate = _replace_candidate_options(
+                    candidate, outcome.options, outcome.answer
+                )
+
             # Phase 4: 后处理
             source_texts = list(material.subtitle_sentences)
             pp = run_postprocess(