refactor: extract grounded selector application into helper

将 Phase 3.5 的 grounded selector 逻辑抽成模块级 _apply_grounded_selector，
内部完成策略门控 / 异常捕获 / observation 落库 / hard-fail 拒绝，调用点仅剩
单一失败分支（selector_reason → 重出）。行为不变，测试全绿。

顺带移除 resample_video_interval 死子表达式（硬编码 1，(attempt-1)%1==0 恒真），
使 _process_one_slot 圈复杂度回落至 Task 6 前的 D(25)。_apply_grounded_selector
自身为 A(4)，均满足质量门。

app/question_gen/pipeline_v2.py

@@ -46,6 +46,8 @@ if TYPE_CHECKING:
     from collections.abc import Callable
 
     from app.question_gen.run_store import QuestionGenStore
+    from app.question_gen.sampler_v2 import MaterialContext
+    from app.question_gen.strategy import TaskTypeStrategy
     from app.tree.index import TreeIndex
     from core.protocols import LLMProvider, VLMProvider
 
@@ -341,6 +343,80 @@ def _replace_candidate_options(
     )
 
 
+async def _apply_grounded_selector(
+    candidate: CandidateQuestion,
+    strategy: TaskTypeStrategy,
+    material: MaterialContext,
+    vlm: VLMProvider,
+    config: PipelineConfig,
+    store: QuestionGenStore,
+    item_id: str,
+    slot_id: str,
+    attempt: int,
+    *,
+    session_id: str,
+) -> tuple[CandidateQuestion | None, str | None]:
+    """对 AR 候选跑 grounded selector，落观测，返回 (candidate, reject_reason)。
+
+    完整拥有 Phase 3.5 的分流控制流，使调用方仅需单一失败分支：
+      - 非 AR 策略（`uses_grounded_selector` 为假）：直接放行，返回 (candidate, None)。
+      - 成功：返回 (重组后的 candidate, None)。
+      - selector 异常（`build_grounded_options` 内部抛 ValueError/FileNotFoundError）：
+        仅置 reason，不落 rejected，返回 (None, "selector_error: ...")。
+      - hard-fail：落 observation + mark_item_rejected，返回 (None, reason)。
+
+    observation 无论成败都落库（供 EOB 退化观测与调参）。
+
+    参数:
+        candidate: 待重组的候选题目。
+        strategy: 题型策略（决定是否走 grounded 路径）。
+        material: 采样素材（提供 frame_paths / subtitles）。
+        vlm: VLM 调用端口。
+        config: 管线配置（提供 selector 三参）。
+        store: 日志记录器（落 selector 观测与拒绝态）。
+        item_id: 当前 item 的唯一 ID。
+        slot_id: slot 标识（日志）。
+        attempt: 当前重出轮次（日志）。
+        session_id: 遥测会话 ID。
+
+    返回:
+        (candidate, None) 放行；(None, reject_reason) 要求调用方重出。
+    """
+    if not strategy.uses_grounded_selector:
+        return candidate, None
+
+    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_id, attempt, e)
+        return None, f"selector_error: {e}"
+
+    # 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:
+        reason = "grounded 干扰项不足（selector 硬失败）"
+        store.mark_item_rejected(item_id, reason)
+        logger.info("slot {} selector 硬失败 (attempt {})", slot_id, attempt)
+        return None, reason
+
+    return _replace_candidate_options(candidate, outcome.options, outcome.answer), None
+
+
 # ---------------------------------------------------------------------------
 # 单 Slot 处理（重出循环）
 # ---------------------------------------------------------------------------
@@ -393,8 +469,6 @@ async def _process_one_slot(
     返回:
         GeneratedQuestion（通过全部检查）或 None（重出耗尽）。
     """
-    resample_video_interval = 1
-
     async with sem:
         strategy = get_strategy(slot.task_type)
         sub_pattern = strategy.select_sub_pattern(rng)
@@ -404,8 +478,8 @@ async def _process_one_slot(
         current_video_id = slot.video_id
 
         for attempt in range(1, config.retry_limit + 1):
-            # 连续失败 resample_video_interval 次后换视频
-            if attempt > 1 and (attempt - 1) % resample_video_interval == 0 and all_trees:
+            # 每次重试都换视频（连续失败即切换）
+            if attempt > 1 and all_trees:
                 alt_ids = [v for v in all_trees if v != current_video_id]
                 if alt_ids:
                     current_video_id = rng.choice(alt_ids)
@@ -488,50 +562,22 @@ 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 3.5: grounded selector（仅 AR 路径；helper 内部完成分流与落库）
+            candidate, selector_reason = await _apply_grounded_selector(
+                candidate,
+                strategy,
+                material,
+                vlm,
+                config,
+                store,
+                item_id,
+                slot.slot_id,
+                attempt,
+                session_id=session_id,
+            )
+            if selector_reason is not None:
+                prev_reason = selector_reason
+                continue
 
             # Phase 4: 后处理
             source_texts = list(material.subtitle_sentences)