feat: wire subject transforms into NatsServer message delivery path

src/NATS.Server/NatsServer.cs

@@ -32,6 +32,7 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     private readonly Account _systemAccount;
     private readonly SslServerAuthenticationOptions? _sslOptions;
     private readonly TlsRateLimiter? _tlsRateLimiter;
+    private readonly SubjectTransform[] _subjectTransforms;
     private Socket? _listener;
     private MonitorServer? _monitorServer;
     private ulong _nextClientId;
@@ -297,6 +298,27 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
                 _tlsRateLimiter = new TlsRateLimiter(options.TlsRateLimit);
         }
 
+        // Compile subject transforms
+        if (options.SubjectMappings is { Count: > 0 })
+        {
+            var transforms = new List<SubjectTransform>();
+            foreach (var (source, dest) in options.SubjectMappings)
+            {
+                var t = SubjectTransform.Create(source, dest);
+                if (t != null)
+                    transforms.Add(t);
+                else
+                    _logger.LogWarning("Invalid subject mapping: {Source} -> {Dest}", source, dest);
+            }
+            _subjectTransforms = transforms.ToArray();
+            if (_subjectTransforms.Length > 0)
+                _logger.LogInformation("Compiled {Count} subject transform(s)", _subjectTransforms.Length);
+        }
+        else
+        {
+            _subjectTransforms = [];
+        }
+
         BuildCachedInfo();
     }
 
@@ -512,6 +534,20 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     public void ProcessMessage(string subject, string? replyTo, ReadOnlyMemory<byte> headers,
         ReadOnlyMemory<byte> payload, NatsClient sender)
     {
+        // Apply subject transforms
+        if (_subjectTransforms.Length > 0)
+        {
+            foreach (var transform in _subjectTransforms)
+            {
+                var mapped = transform.Apply(subject);
+                if (mapped != null)
+                {
+                    subject = mapped;
+                    break; // First matching transform wins
+                }
+            }
+        }
+
         var subList = sender.Account?.SubList ?? _globalAccount.SubList;
         var result = subList.Match(subject);
         var delivered = false;