perf: optimize MQTT cross-protocol path (0.30x → 0.78x Go)

Replace per-message async fire-and-forget with direct-buffer write loop
mirroring NatsClient pattern: SpinLock-guarded buffer append, double-
buffer swap, single WriteAsync per batch.

- MqttConnection: add _directBuf/_writeBuf + RunMqttWriteLoopAsync
- MqttConnection: add EnqueuePublishNoFlush (zero-alloc PUBLISH format)
- MqttPacketWriter: add WritePublishTo(Span<byte>) + MeasurePublish
- MqttTopicMapper: add NatsToMqttBytes with bounded ConcurrentDictionary
- MqttNatsClientAdapter: synchronous SendMessageNoFlush + SignalFlush
- Skip FlushAsync on plain TCP sockets (TCP auto-flushes)

tests/NATS.Server.Benchmark.Tests/Infrastructure/MqttServerFixture.cs

@@ -0,0 +1,93 @@
+using NATS.Client.Core;
+
+namespace NATS.Server.Benchmark.Tests.Infrastructure;
+
+/// <summary>
+/// Starts both a Go and .NET NATS server with MQTT and JetStream enabled for MQTT benchmarks.
+/// Shared across all tests in the "Benchmark-Mqtt" collection.
+/// </summary>
+public sealed class MqttServerFixture : IAsyncLifetime
+{
+    private GoServerProcess? _goServer;
+    private DotNetServerProcess? _dotNetServer;
+    private string? _goStoreDir;
+    private string? _dotNetStoreDir;
+
+    public int GoNatsPort => _goServer?.Port ?? throw new InvalidOperationException("Go server not started");
+    public int GoMqttPort { get; private set; }
+    public int DotNetNatsPort => _dotNetServer?.Port ?? throw new InvalidOperationException(".NET server not started");
+    public int DotNetMqttPort { get; private set; }
+    public bool GoAvailable => _goServer is not null;
+
+    public async Task InitializeAsync()
+    {
+        DotNetMqttPort = PortAllocator.AllocateFreePort();
+        _dotNetStoreDir = Path.Combine(Path.GetTempPath(), "nats-bench-dotnet-mqtt-" + Guid.NewGuid().ToString("N")[..8]);
+        Directory.CreateDirectory(_dotNetStoreDir);
+
+        var dotNetConfig = $$"""
+            jetstream {
+                store_dir: "{{_dotNetStoreDir}}"
+                max_mem_store: 64mb
+                max_file_store: 256mb
+            }
+            mqtt {
+                listen: 127.0.0.1:{{DotNetMqttPort}}
+            }
+            """;
+
+        _dotNetServer = new DotNetServerProcess(dotNetConfig);
+        var dotNetTask = _dotNetServer.StartAsync();
+
+        if (GoServerProcess.IsAvailable())
+        {
+            GoMqttPort = PortAllocator.AllocateFreePort();
+            _goStoreDir = Path.Combine(Path.GetTempPath(), "nats-bench-go-mqtt-" + Guid.NewGuid().ToString("N")[..8]);
+            Directory.CreateDirectory(_goStoreDir);
+
+            var goConfig = $$"""
+                jetstream {
+                    store_dir: "{{_goStoreDir}}"
+                    max_mem_store: 64mb
+                    max_file_store: 256mb
+                }
+                mqtt {
+                    listen: 127.0.0.1:{{GoMqttPort}}
+                }
+                """;
+
+            _goServer = new GoServerProcess(goConfig);
+            await Task.WhenAll(dotNetTask, _goServer.StartAsync());
+        }
+        else
+        {
+            await dotNetTask;
+        }
+    }
+
+    public async Task DisposeAsync()
+    {
+        if (_goServer is not null)
+            await _goServer.DisposeAsync();
+        if (_dotNetServer is not null)
+            await _dotNetServer.DisposeAsync();
+
+        CleanupDir(_goStoreDir);
+        CleanupDir(_dotNetStoreDir);
+    }
+
+    public NatsConnection CreateGoNatsClient()
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{GoNatsPort}" });
+
+    public NatsConnection CreateDotNetNatsClient()
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{DotNetNatsPort}" });
+
+    private static void CleanupDir(string? dir)
+    {
+        if (dir is not null && Directory.Exists(dir))
+        {
+            try { Directory.Delete(dir, recursive: true); }
+            catch { /* best-effort cleanup */ }
+        }
+    }
+}