feat: add benchmark test project for Go vs .NET server comparison

Side-by-side performance benchmarks using NATS.Client.Core against both
servers on ephemeral ports. Includes core pub/sub, request/reply latency,
and JetStream throughput tests with comparison output and
benchmarks_comparison.md results. Also fixes timestamp flakiness in
StoreInterfaceTests by using explicit timestamps.

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

@@ -0,0 +1,47 @@
+using NATS.Client.Core;
+
+namespace NATS.Server.Benchmark.Tests.Infrastructure;
+
+/// <summary>
+/// Starts both a Go and .NET NATS server for core pub/sub benchmarks.
+/// Shared across all tests in the "Benchmark-Core" collection.
+/// </summary>
+public sealed class CoreServerPairFixture : IAsyncLifetime
+{
+    private GoServerProcess? _goServer;
+    private DotNetServerProcess? _dotNetServer;
+
+    public int GoPort => _goServer?.Port ?? throw new InvalidOperationException("Go server not started");
+    public int DotNetPort => _dotNetServer?.Port ?? throw new InvalidOperationException(".NET server not started");
+    public bool GoAvailable => _goServer is not null;
+
+    public async Task InitializeAsync()
+    {
+        _dotNetServer = new DotNetServerProcess();
+        var dotNetTask = _dotNetServer.StartAsync();
+
+        if (GoServerProcess.IsAvailable())
+        {
+            _goServer = new GoServerProcess();
+            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();
+    }
+
+    public NatsConnection CreateGoClient()
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{GoPort}" });
+
+    public NatsConnection CreateDotNetClient()
+        => new(new NatsOpts { Url = $"nats://127.0.0.1:{DotNetPort}" });
+}