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。

tests/integration/test_pipeline_v2.py

@@ -194,8 +194,15 @@ class MockVLM:
         parent_call_id: str | None = None,
     ) -> LLMResponse:
         prompt_text = str(messages)
+        system_text = messages[0].get("content", "") if messages else ""
         if "verdict" in prompt_text.lower():
             return _make_llm_response(self._gate_response)
+        if "distractor" in system_text.lower() and "grader" not in system_text.lower():
+            # 候选池请求：返回 4 个 grounded 干扰项
+            return _make_llm_response('{"distractors": ["蒸", "煮", "炸", "烤"]}')
+        if "grader" in system_text.lower():
+            # 打分请求：正解高分、3 个落区间、1 个负空间
+            return _make_llm_response('{"scores": [0.90, 0.80, 0.70, 0.60, 0.20]}')
         idx = min(self._gen_count, len(self._responses) - 1)
         self._gen_count += 1
         return _make_llm_response(self._responses[idx])