feat: port session 10 — Server Core Runtime, Accept Loops & Listeners

Ports server/server.go lines 2577–4782 (~1,881 Go LOC), implementing ~97
features (IDs 3051–3147) across three new partial-class files.

New files:
- NatsServer.Lifecycle.cs: Shutdown, WaitForShutdown, RemoveClient,
  SendLDMToClients, LameDuckMode, LDMClientByID, rate-limit logging,
  DisconnectClientByID, SendAsyncInfoToClients
- NatsServer.Listeners.cs: AcceptLoop, GetServerListener, InProcessConn,
  AcceptConnections, GenerateInfoJson, CopyInfo, CreateClient/Ex/InProcess,
  StartMonitoring (HTTP/HTTPS), AddConnectURLs/RemoveConnectURLs,
  TlsVersion/TlsVersionFromString, GetClientConnectURLs, ResolveHostPorts,
  PortsInfo/PortFile/LogPorts, ReadyForListeners, GetRandomIP, AcceptError
- Internal/WaitGroup.cs: Go-style WaitGroup using TaskCompletionSource

Modified:
- Auth/AuthTypes.cs: Account now implements INatsAccount (stub)
- NatsServerTypes.cs: ServerInfo.ShallowClone(), removed duplicate RefCountedUrlSet
- NatsServer.cs: _info promoted to internal for test access
- Properties/AssemblyInfo.cs: InternalsVisibleTo(DynamicProxyGenAssembly2)
- ServerTests.cs: 20 new session-10 unit tests (GenerateInfoJson, TlsVersion,
  CopyInfo, GetRandomIP — Test IDs 2895, 2906)

All 565 unit tests + 1 integration test pass.

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

@@ -13,6 +13,8 @@
 //
 // Adapted from server/auth.go (type definitions) in the NATS server Go source.
 
+using ZB.MOM.NatsNet.Server;
+
 namespace ZB.MOM.NatsNet.Server.Auth;
 
 /// <summary>
@@ -170,7 +172,7 @@ public class RoutePermissions
 /// Stub for Account type. Full implementation in session 11.
 /// Mirrors Go <c>Account</c> struct.
 /// </summary>
-public class Account
+public class Account : INatsAccount
 {
     public string Name         { get; set; } = string.Empty;
 
@@ -184,6 +186,27 @@ public class Account
     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) { }
 }