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_verify.py

@@ -55,9 +55,9 @@ class TestVerifyTree:
                 visible_text=["Nike", "2024"],
                 spatial_layout="居中",
                 visual_attributes={},
+                subtitle="the athlete is running fast",
             ),
             timestamp=1.0,
-            subtitle="the athlete is running fast",
         )
         l3_1 = L3Node(
             id="l1_0_l2_0_l3_1",