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/ServerOptions.cs

@@ -109,7 +109,7 @@ public sealed partial class ServerOptions
     public string SystemAccount { get; set; } = string.Empty;
     public bool NoSystemAccount { get; set; }
     /// <summary>Parsed account objects from config. Mirrors Go opts.Accounts.</summary>
-    public List<Auth.Account> Accounts { get; set; } = [];
+    public List<Account> Accounts { get; set; } = [];
     public AuthCalloutOpts? AuthCallout { get; set; }
     public bool AlwaysEnableNonce { get; set; }
     public List<User>? Users { get; set; }