feat: session B — auth implementation + signals (26 stubs complete)

Implement ConfigureAuthorization, CheckAuthentication, and full auth
dispatch in NatsServer.Auth.cs; add HandleSignals in NatsServer.Signals.cs;
extend AuthHandler with GetAuthErrClosedState, ValidateProxies,
GetTlsAuthDcs, CheckClientTlsCertSubject, ProcessUserPermissionsTemplate;
add ReadOperatorJwt/ValidateTrustedOperators to JwtProcessor; add
AuthCallout stub; add auth accessor helpers to ClientConnection; add
NATS.NKeys package for NKey signature verification; 12 new tests pass.

dotnet/src/ZB.MOM.NatsNet.Server/NatsServer.Signals.cs

@@ -0,0 +1,82 @@
+// Copyright 2012-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Adapted from server/signal.go in the NATS server Go source.
+
+using System.Runtime.InteropServices;
+
+namespace ZB.MOM.NatsNet.Server;
+
+/// <summary>
+/// OS signal handling for <see cref="NatsServer"/>.
+/// Mirrors Go <c>signal.go</c>.
+/// </summary>
+public sealed partial class NatsServer
+{
+    private PosixSignalRegistration? _sigHup;
+    private PosixSignalRegistration? _sigTerm;
+    private PosixSignalRegistration? _sigInt;
+
+    /// <summary>
+    /// Registers OS signal handlers (SIGHUP, SIGTERM, SIGINT).
+    /// On Windows, falls back to <see cref="Console.CancelKeyPress"/>.
+    /// Mirrors Go <c>Server.handleSignals</c>.
+    /// </summary>
+    internal void HandleSignals()
+    {
+        if (GetOpts()?.NoSigs == true) return;
+
+        if (OperatingSystem.IsWindows())
+        {
+            Console.CancelKeyPress += (_, e) =>
+            {
+                e.Cancel = true;
+                Noticef("Caught interrupt signal, shutting down...");
+                _ = ShutdownAsync();
+            };
+            return;
+        }
+
+        // SIGHUP — reload configuration
+        _sigHup = PosixSignalRegistration.Create(PosixSignal.SIGHUP, ctx =>
+        {
+            ctx.Cancel = true;
+            Noticef("Trapped SIGHUP signal, reloading configuration...");
+            try { Reload(); }
+            catch (Exception ex) { Errorf("Config reload failed: {0}", ex.Message); }
+        });
+
+        // SIGTERM — graceful shutdown
+        _sigTerm = PosixSignalRegistration.Create(PosixSignal.SIGTERM, ctx =>
+        {
+            ctx.Cancel = true;
+            Noticef("Trapped SIGTERM signal, shutting down...");
+            _ = ShutdownAsync();
+        });
+
+        // SIGINT — interrupt (Ctrl+C)
+        _sigInt = PosixSignalRegistration.Create(PosixSignal.SIGINT, ctx =>
+        {
+            ctx.Cancel = true;
+            Noticef("Trapped SIGINT signal, shutting down...");
+            _ = ShutdownAsync();
+        });
+    }
+
+    private void DisposeSignalHandlers()
+    {
+        _sigHup?.Dispose();
+        _sigTerm?.Dispose();
+        _sigInt?.Dispose();
+    }
+}