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/CorePubSub/SinglePublisherThroughputTests.cs

@@ -0,0 +1,63 @@
+using NATS.Client.Core;
+using NATS.Server.Benchmark.Tests.Harness;
+using NATS.Server.Benchmark.Tests.Infrastructure;
+using Xunit.Abstractions;
+
+namespace NATS.Server.Benchmark.Tests.CorePubSub;
+
+[Collection("Benchmark-Core")]
+public class SinglePublisherThroughputTests(CoreServerPairFixture fixture, ITestOutputHelper output)
+{
+    private readonly BenchmarkRunner _runner = new() { WarmupCount = 1_000, MeasurementCount = 100_000 };
+
+    [Fact]
+    [Trait("Category", "Benchmark")]
+    public async Task PubNoSub_16B()
+    {
+        const int payloadSize = 16;
+        var payload = new byte[payloadSize];
+        const string subject = "bench.pub.nosub.16";
+
+        var dotnetResult = await RunThroughput("Single Publisher (16B)", "DotNet", subject, payload, fixture.CreateDotNetClient);
+
+        if (fixture.GoAvailable)
+        {
+            var goResult = await RunThroughput("Single Publisher (16B)", "Go", subject, payload, fixture.CreateGoClient);
+            BenchmarkResultWriter.WriteComparison(output, goResult, dotnetResult);
+        }
+        else
+        {
+            BenchmarkResultWriter.WriteSingle(output, dotnetResult);
+        }
+    }
+
+    [Fact]
+    [Trait("Category", "Benchmark")]
+    public async Task PubNoSub_128B()
+    {
+        const int payloadSize = 128;
+        var payload = new byte[payloadSize];
+        const string subject = "bench.pub.nosub.128";
+
+        var dotnetResult = await RunThroughput("Single Publisher (128B)", "DotNet", subject, payload, fixture.CreateDotNetClient);
+
+        if (fixture.GoAvailable)
+        {
+            var goResult = await RunThroughput("Single Publisher (128B)", "Go", subject, payload, fixture.CreateGoClient);
+            BenchmarkResultWriter.WriteComparison(output, goResult, dotnetResult);
+        }
+        else
+        {
+            BenchmarkResultWriter.WriteSingle(output, dotnetResult);
+        }
+    }
+
+    private async Task<BenchmarkResult> RunThroughput(string name, string serverType, string subject, byte[] payload, Func<NatsConnection> createClient)
+    {
+        await using var client = createClient();
+        await client.ConnectAsync();
+
+        return await _runner.MeasureThroughputAsync(name, serverType, payload.Length,
+            async _ => await client.PublishAsync(subject, payload));
+    }
+}