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/unit/test_subtitle.py

@@ -112,7 +112,11 @@ class TestVoronoiAssign:
         entries = [SRTEntry(1.0, 3.0, "hello"), SRTEntry(5.0, 7.0, "world")]
         assign_subtitles_voronoi(index, entries)
 
-        assert l3_0.subtitle is not None
-        assert "hello" in l3_0.subtitle
-        assert l3_1.subtitle is not None
-        assert "world" in l3_1.subtitle
+        assert l3_0.card.subtitle != ""
+        assert "hello" in l3_0.card.subtitle
+        assert l3_1.card.subtitle != ""
+        assert "world" in l3_1.card.subtitle
+        # L2 字幕应聚合 L3 子节点字幕
+        assert l2.card.subtitle != ""
+        assert "hello" in l2.card.subtitle
+        assert "world" in l2.card.subtitle