test(vtt-thumbnail): [PKG-VTT-THUMB-02] reproduce parsing and lifecycle failures

refactor/tasks.json

@@ -1876,11 +1876,14 @@
         "ENG-03",
         "ENG-05"
       ],
-      "status": "todo",
+      "status": "done",
       "risk": "M",
       "deliverable": "VTT 解析、xywh、相对 URL、时间边界、请求失败和定位",
       "acceptance": "旧版本行为可重跑，成功/失败/切源/销毁有必要断言",
-      "evidence": []
+      "evidence": [
+        "baselines/vtt-thumbnail-failures.json",
+        "changes/2026-09-13-PKG-VTT-THUMB-02-failures.md"
+      ]
     },
     {
       "id": "PKG-VTT-THUMB-03",