refactor(vtt-thumbnail): [PKG-VTT-THUMB-04] preserve legacy types and expose async runtime entry

refactor/tasks.json

@@ -1927,7 +1927,9 @@
       "acceptance": "严格类型检查、旧消费样例通过；声明路径/导出和同步异步兼容",
       "evidence": [
         "changes/2026-09-13-PKG-VTT-THUMB-04-runtime-types.md",
-        "baselines/vtt-thumbnail-runtime-types.json"
+        "baselines/vtt-thumbnail-runtime-types.json",
+        "changes/2026-09-13-PKG-VTT-THUMB-04-public-types.md",
+        "baselines/vtt-thumbnail-public-types.json"
       ]
     },
     {