feat: port session 09 — Server Core Init & Config

Port server/server.go account management and initialization (~1950 LOC):

- NatsServer.cs: full server struct fields (atomic counters, locks, maps,
  stubs for gateway/websocket/mqtt/ocsp/leafnode)
- NatsServer.Init.cs: factory methods (New/NewServer/NewServerFromConfig),
  compression helpers (ValidateAndNormalizeCompressionOption,
  SelectCompressionMode, SelectS2AutoModeBasedOnRtt, CompressOptsEqual),
  cluster-name management, validation (ValidateCluster, ValidatePinnedCerts,
  ValidateOptions), trusted-key processing, CLI helpers, running-state checks,
  and Start() stub
- NatsServer.Accounts.cs: account management (ConfigureAccounts,
  LookupOrRegisterAccount, RegisterAccount, SetSystemAccount,
  SetDefaultSystemAccount, SetSystemAccountInternal, CreateInternalClient*,
  ShouldTrackSubscriptions, RegisterAccountNoLock, SetAccountSublist,
  SetRouteInfo, LookupAccount, LookupOrFetchAccount, UpdateAccount,
  UpdateAccountWithClaimJwt, FetchRawAccountClaims, FetchAccountClaims,
  VerifyAccountClaims, FetchAccountFromResolver, GlobalAccountOnly,
  StandAloneMode, ConfiguredRoutes, ActivePeers, ComputeRoutePoolIdx)
- NatsServerTypes.cs: ServerInfo, ServerStats, NodeInfo, ServerProtocol,
  CompressionMode constants, AccountClaims stub, InternalState stub, and
  cross-session stubs for JetStream/gateway/websocket/mqtt/ocsp
- AuthTypes.cs: extend Account stub with Issuer, ClaimJwt, RoutePoolIdx,
  Incomplete, Updated, Sublist, Server fields, and IsExpired()
- ServerOptions.cs: add Accounts property (List<Account>)
- ServerTests.cs: 38 standalone tests (IDs 2866, 2882, plus compression
  and validation helpers); server-dependent tests marked n/a

Features: 77 complete (IDs 2974–3050)
Tests: 2 complete (2866, 2882); 18 n/a (server-dependent)
All tests: 545 unit + 1 integration pass

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

@@ -167,10 +167,23 @@ public class RoutePermissions
 }
 
 /// <summary>
-/// Stub for Account type. Full implementation in later sessions.
+/// Stub for Account type. Full implementation in session 11.
 /// Mirrors Go <c>Account</c> struct.
 /// </summary>
 public class Account
 {
-    public string Name { get; set; } = string.Empty;
+    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
+
+    /// <summary>Returns true if this account's JWT has expired. Stub — full impl in session 11.</summary>
+    public bool IsExpired() => false;
 }