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_search_tools.py

@@ -111,10 +111,10 @@ def _make_test_tree() -> TreeIndex:
             visible_text=[],
             spatial_layout="center",
             visual_attributes={},
+            subtitle="test subtitle text",
         ),
         timestamp=10.0,
         frame_path="frames/L1_000_L2_000_L3_000.jpg",
-        subtitle="test subtitle text",
     )
     l2 = L2Node(
         id="vid_L1_000_L2_000",

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

tests/unit/test_tree_environment.py

@@ -31,10 +31,10 @@ def _make_test_index() -> TreeIndex:
             ["Nike"],
             "居中",
             {"lighting": "明亮"},
+            subtitle="he is running",
         ),
         timestamp=1.0,
         frame_path="frames/L1_000_L2_000_L3_000.jpg",
-        subtitle="he is running",
     )
     l3_1 = L3Node(
         id="vid_L1_000_L2_000_L3_001",

tests/unit/test_tree_index.py

@@ -130,9 +130,9 @@ class TestNodes:
         node = _make_l3()
         assert node.embedding is None
 
-    def test_l3_subtitle_default_none(self):
+    def test_l3_card_subtitle_default_empty(self):
         node = _make_l3()
-        assert node.subtitle is None
+        assert node.card.subtitle == ""
 
 
 class TestTreeIndex:

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",