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。

Client/Assets/Scripts/Services/PlayerData/PlayerKVRuntimeSelection.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Globalization;
+
+namespace game.PlayerDataStorage
+{
+    /// <summary>
+    /// Immutable process-start selection. Accounts 101 and 102 are the reviewed first cohort;
+    /// expanding it requires a new complete migration design review.
+    /// </summary>
+    internal sealed class PlayerKVRuntimeSelection
+    {
+        internal const string FormalApiUrl = "http://s101.local:18080/";
+
+        internal bool IsKV { get; }
+        internal Uri Api { get; }
+
+        private PlayerKVRuntimeSelection(bool isKV, Uri api)
+        {
+            IsKV = isKV;
+            Api = api;
+        }
+
+        internal static PlayerKVRuntimeSelection Capture(string backend, string api, bool agg)
+        {
+            if (backend == null || backend == "playfab")
+                return new PlayerKVRuntimeSelection(false, null);
+            if (backend != "kv")
+                throw new ArgumentException("Unsupported save backend.", nameof(backend));
+            if (agg)
+                throw new InvalidOperationException("KV login is not enabled for AGG channels.");
+            if (!string.Equals(api, FormalApiUrl, StringComparison.Ordinal))
+                throw new ArgumentException("KV API address does not match the reviewed endpoint.", nameof(api));
+            return new PlayerKVRuntimeSelection(true, new Uri(FormalApiUrl));
+        }
+
+        internal bool TryAdmitPlayer(string requestedId, out string canonicalId)
+        {
+            canonicalId = null;
+            if (!IsKV || !long.TryParse(requestedId, NumberStyles.None, CultureInfo.InvariantCulture, out var id) ||
+                (id != 101 && id != 102)) return false;
+            canonicalId = id.ToString(CultureInfo.InvariantCulture);
+            return true;
+        }
+    }
+}