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>

app/tree/verify.py

@@ -97,8 +97,8 @@ def _collect_l3_text(l2_node: L2Node) -> str:
     for l3 in l2_node.children:
         parts.append(l3.card.frame_summary)
         parts.extend(l3.card.visible_text)
-        if l3.subtitle:
-            parts.append(l3.subtitle)
+        if l3.card.subtitle:
+            parts.append(l3.card.subtitle)
     return "\n".join(parts)
 
 
@@ -139,8 +139,8 @@ def _collect_descendant_text_corpus(l1_node: L1Node) -> str:
         for l3 in l2.children:
             parts.append(l3.card.frame_summary)
             parts.extend(l3.card.visible_text)
-            if l3.subtitle:
-                parts.append(l3.subtitle)
+            if l3.card.subtitle:
+                parts.append(l3.card.subtitle)
     return "\n".join(parts)
 
 
@@ -221,6 +221,7 @@ def _verify_l2(l2: L2Node, stats: VerifyStats) -> None:
         visible_text=kept_vt,
         spatial_relations=old_card.spatial_relations,
         state_changes=old_card.state_changes,
+        subtitle=old_card.subtitle,
     )