fix(question_gen): sample_anchor Codex 审查修正

- C1/C2: Temporal Reasoning ≥3 L2 + Object Reasoning ≥2 L2 下限检查
- I1: L2 题型子帧不足时 ValueError
- I2: L3 过滤无 frame_path 的节点
- I3/I4: distractor_texts 扩展到整棵树范围
- I5-I7: 测试补强 Information Synopsis/Temporal Reasoning/Object Reasoning
- M1: Spatial Reasoning 测试断言 spatial_layout

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

tests/unit/test_synthesizer.py

@@ -185,15 +185,15 @@ class TestSampleAnchor:
         """Information Synopsis 必须使用目标 L1 下所有 L2 子节点。"""
         tree, _vid = _load_test_tree()
         ctx = sample_anchor(tree, "Information Synopsis", set(), random.Random(42))
-        # 至少应有帧（每个 L2 取一帧代表）
-        total_l2 = sum(len(r.children) for r in tree.roots)
-        assert len(ctx.frame_paths) >= min(total_l2, 1)
+        # 找到被选中的 L1，验证 frame_paths 数量 == 该 L1 下全部 L2 数量
+        chosen_l1 = next(r for r in tree.roots if r.id == ctx.node_id)
+        assert len(ctx.frame_paths) == len(chosen_l1.children)
 
     def test_l1_type_l2_nodes_in_time_order(self) -> None:
-        """L1 题型（Temporal Reasoning）的 card_text 应有实质内容。"""
+        """Temporal Reasoning 应返回 >=3 帧且 card_text 有实质内容。"""
         tree, _vid = _load_test_tree()
         ctx = sample_anchor(tree, "Temporal Reasoning", set(), random.Random(42))
-        assert len(ctx.frame_paths) >= 1
+        assert len(ctx.frame_paths) >= 3
         assert len(ctx.card_text) > 20
 
     def test_used_node_ids_excluded(self) -> None:
@@ -219,12 +219,13 @@ class TestSampleAnchor:
         """Object Reasoning (L1-L2) 应选 2-3 个 L2 并按时间排序。"""
         tree, _vid = _load_test_tree()
         ctx = sample_anchor(tree, "Object Reasoning", set(), random.Random(42))
-        assert 1 <= len(ctx.frame_paths) <= 3
+        assert 2 <= len(ctx.frame_paths) <= 3
         assert len(ctx.card_text) > 10
 
     def test_spatial_reasoning_context_fields(self) -> None:
-        """Spatial Reasoning 的 card_text 应包含 spatial_layout 内容。"""
+        """Spatial Reasoning 的 card_text 应包含 spatial_layout 字段。"""
         tree, _vid = _load_test_tree()
         ctx = sample_anchor(tree, "Spatial Reasoning", set(), random.Random(42))
-        # context_fields 包含 spatial_layout，card_text 应含有该字段内容
+        # context_fields 包含 spatial_layout，card_text 必须出现该字段名
+        assert "spatial_layout" in ctx.card_text
         assert len(ctx.card_text) > 20