feat: port session 11 — Accounts & Directory JWT Store

- Account: full Account class (200 features) with subject mappings,
  connection counting, export/import checks, expiration timers
- DirJwtStore: directory-based JWT storage with sharding and expiry
- AccountResolver: IAccountResolver, MemoryAccountResolver,
  UrlAccountResolver, DirAccountResolver, CacheDirAccountResolver
- AccountTypes: all supporting types (AccountLimits, SConns, ExportMap,
  ImportMap, ServiceExport, StreamExport, ServiceLatency, etc.)
- 34 unit tests (599 total), 234 features complete (IDs 150-349, 793-826)

dotnet/src/ZB.MOM.NatsNet.Server/Auth/AuthTypes.cs

@@ -168,45 +168,5 @@ public class RoutePermissions
     public SubjectPermission? Export { get; set; }
 }
 
-/// <summary>
-/// Stub for Account type. Full implementation in session 11.
-/// Mirrors Go <c>Account</c> struct.
-/// </summary>
-public class Account : INatsAccount
-{
-    public string Name         { get; set; } = string.Empty;
-
-    // Fields required by session 09 account management (NatsServer.Accounts.cs).
-    // Full implementation in session 11.
-    public string   Issuer     { get; set; } = string.Empty;
-    internal string ClaimJwt   { get; set; } = string.Empty;
-    internal int    RoutePoolIdx { get; set; }
-    internal bool   Incomplete { get; set; }
-    internal DateTime Updated  { get; set; }
-    internal ZB.MOM.NatsNet.Server.Internal.DataStructures.SubscriptionIndex? Sublist { get; set; }
-    internal object? Server    { get; set; }  // INatsServer — avoids circular reference
-
-    // INatsAccount — stub implementations until session 11 (accounts.go).
-    bool INatsAccount.IsValid                         => true;
-    bool INatsAccount.MaxTotalConnectionsReached()    => false;
-    bool INatsAccount.MaxTotalLeafNodesReached()      => false;
-    int  INatsAccount.AddClient(ClientConnection c)   => 0;
-    int  INatsAccount.RemoveClient(ClientConnection c) => 0;
-
-    /// <summary>Returns true if this account's JWT has expired. Stub — full impl in session 11.</summary>
-    public bool IsExpired() => false;
-
-    /// <summary>
-    /// Returns the total number of subscriptions across all clients in this account.
-    /// Stub — full implementation in session 11.
-    /// Mirrors Go <c>Account.TotalSubs()</c>.
-    /// </summary>
-    public int TotalSubs() => 0;
-
-    /// <summary>
-    /// Notifies leaf nodes of a subscription change.
-    /// Stub — full implementation in session 15.
-    /// Mirrors Go <c>Account.updateLeafNodes()</c>.
-    /// </summary>
-    internal void UpdateLeafNodes(object sub, int delta) { }
-}
+// Account stub removed — full implementation is in Accounts/Account.cs
+// in the ZB.MOM.NatsNet.Server namespace.