refactor(tree): subtitle 迁入 L3Card/L2Card + 建树管线修正

- L3Card/L2Card 新增 subtitle: str 字段（L1Card 不加）
- L3Node 移除 subtitle 字段（数据迁入 Card）
- assign_subtitles_voronoi 改写 Card.subtitle + L2 聚合
- _collect_card_strings 增加 skip_fields 排除 subtitle
- _node_full_text/_node_anchored_text 保持 字幕:/[sN] 语义
- get_subtitle 读 Card.subtitle（L2/L3）
- verify.py/synthesizer.py: l3.subtitle → l3.card.subtitle
- 迁移脚本 tools/migrate_subtitle_to_card.py（幂等，300 棵树已迁移）
- 9→6 个测试文件适配（3 个无需改动）

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

tests/integration/test_tree_build_e2e.py

@@ -97,10 +97,10 @@ class TestTreeModuleE2E:
             SRTEntry(start=5.0, end=7.0, text="The crowd goes wild!"),
         ]
         assign_subtitles_voronoi(index, srt_entries)
-        assert l3_0.subtitle is not None
-        assert "sprints" in l3_0.subtitle
-        assert l3_1.subtitle is not None
-        assert "crowd" in l3_1.subtitle
+        assert l3_0.card.subtitle != ""
+        assert "sprints" in l3_0.card.subtitle
+        assert l3_1.card.subtitle != ""
+        assert "crowd" in l3_1.card.subtitle
 
         # Step 3: TreeEnvironment 查询
         env = TreeEnvironment(index)
@@ -139,8 +139,8 @@ class TestTreeModuleE2E:
 
         assert len(loaded.roots) == 1
         assert loaded.roots[0].card.scene_summary == "百米短跑决赛"
-        assert loaded.roots[0].children[0].children[0].subtitle is not None
-        assert "sprints" in loaded.roots[0].children[0].children[0].subtitle
+        assert loaded.roots[0].children[0].children[0].card.subtitle != ""
+        assert "sprints" in loaded.roots[0].children[0].children[0].card.subtitle
         # verify 的修改也被保留
         assert "幻觉实体XYZ" not in loaded.roots[0].children[0].card.entities