feat(upserver): 保存M2客户端KV迁移至本人统计写检查点

保存独立KV用户服务、固定会话存档与原业务接线，以及C3d资料缓存和C3e本人统计写。
保留原PF/AGG路线、业务公式和时序；榜单读取、好友资料、公告、匹配及首场景闸仍待完成。

验证：ServiceHost两条件各33组646断言；Unity CLI Loop编译0错误269警告。
包含相关方案、评审裁决、测试及状态文档。未操作服务器、正式账号或打包。

CodexOutput/upserver/B3BatchMigration/M2/M2-C3c-KV跨账号实测增量hash.json

@@ -0,0 +1,25 @@
+{
+  "date": "2026-09-30T16:36:07.2771599+08:00",
+  "scope": "Explicit user-authorized read-only cross-account probes; all eight return own data",
+  "items": [
+    {
+      "path": "CodexOutput/upserver/B3BatchMigration/M2/M2-C3c-KV跨账号读取实测-2026-09-30.json",
+      "bytes": 7038,
+      "sha256": "E1F29E4C218D71A8AB872C698EB6F8A5E0DE12D3713283E0BE8D41E5086A2FF5"
+    },
+    {
+      "path": "CodexOutput/upserver/B3BatchMigration/M2/M2-C3c-KV跨账号读取实测结论-2026-09-30.md",
+      "bytes": 2775,
+      "sha256": "89583058E956FFA682C79E60EA12CEF0C2EF736175047B19FD7D6FF5E3C1B2B3"
+    },
+    {
+      "path": "Client/Assets/Scripts/Services/PlayerData/PlayerKVClient.cs",
+      "bytes": 6765,
+      "sha256": "9D2D9FC38A0EE48F2CB0B926BB733621045FE179C5B20E7717B0F975D415D2FE"
+    }
+  ],
+  "count": 3,
+  "formalRequests": 14,
+  "kvWrites": 0,
+  "productionChanged": false
+}