refactor(mediabunny): [PKG-MB-04] checkpoint typed shim and event cleanup

refactor/tasks.json

@@ -3460,11 +3460,15 @@
         "CORE-06",
         "CORE-11"
       ],
-      "status": "todo",
+      "status": "doing",
       "risk": "H",
       "deliverable": "shim 描述符、事件桥、加载状态和操作隔离",
       "acceptance": "公开属性/同步异步返回与事件重入顺序保持",
-      "evidence": []
+      "evidence": [
+        "baselines/mb-coordination-source.json",
+        "changes/2026-09-12-PKG-MB-04-shim-checkpoint.md",
+        "baselines/mb-shim-checkpoint.json"
+      ]
     },
     {
       "id": "PKG-MB-05",