fix: IndexedDB probe 失败后不再永久卡在 loading，重试耗尽后回退到 blocked

- index.js: 新增 reconcileIndexedDbProbeFailureState，后台 probe 失败时先有限重试，耗尽后切到 blocked
- index.js: scheduleIndexedDbGraphProbe 的 .then/.catch 均接入 reconcile 逻辑
- index.js: createGraphLoadUiStatus blocked 文案更新
- ui/panel.js: _getGraphLoadLabel blocked 文案更新，不再误导为元数据未就绪
- tests/graph-persistence.mjs: 新增 manager-unavailable / read-failed 回归
- tests/graph-persistence.mjs: harness 支持 __indexedDbExportSnapshotShouldThrow / __indexedDbGetCurrentDbShouldThrow

ui/panel.js

@@ -9913,7 +9913,7 @@ function _getGraphLoadLabel(loadState = "") {
     case "empty-confirmed":
       return "当前聊天还没有图谱";
     case "blocked":
-      return "聊天元数据未就绪，已暂停图谱写回以保护旧数据";
+      return "当前聊天图谱未能完成 IndexedDB 确认，请稍后重试";
     case "loaded":
       return "聊天图谱已加载";
     case "no-chat":