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/unit/test_pipeline_selector_wiring.py

@@ -0,0 +1,42 @@
+"""selector 织入辅助：正解文本提取 + 分流。"""
+
+import pytest
+
+from app.question_gen.pipeline_v2 import _extract_correct_text
+
+
+def test_extract_correct_text_strips_prefix():
+    options = ("A. 蒸", "B. 炒", "C. 煮", "D. 炸")
+    assert _extract_correct_text(options, "C") == "煮"
+
+
+def test_extract_correct_text_handles_lowercase_answer():
+    options = ("A. run", "B. walk", "C. jump", "D. sit")
+    assert _extract_correct_text(options, "b") == "walk"
+
+
+def test_extract_correct_text_out_of_range_raises():
+    options = ("A. a", "B. b", "C. c", "D. d")
+    with pytest.raises(ValueError):
+        _extract_correct_text(options, "E")
+
+
+def test_replace_candidate_options():
+    from app.question_gen.generator_v2 import CandidateQuestion
+    from app.question_gen.pipeline_v2 import _replace_candidate_options
+
+    c = CandidateQuestion(
+        question_id="q",
+        video_id="v",
+        task_type="Action Recognition",
+        skill_target="M1_AR",
+        question="?",
+        options=("A. a", "B. b", "C. c", "D. d"),
+        answer="A",
+        source_nodes=("n1",),
+        difficulty="hard",
+    )
+    new = _replace_candidate_options(c, ("A. 蒸", "B. 炒", "C. 煮", "D. 炸"), "A")
+    assert new.options == ("A. 蒸", "B. 炒", "C. 煮", "D. 炸")
+    assert new.question == "?"  # 其余字段不变
+    assert new.source_nodes == ("n1",)