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/PortAllocator.cs

@@ -0,0 +1,14 @@
+using System.Net;
+using System.Net.Sockets;
+
+namespace NATS.Server.Benchmark.Tests.Infrastructure;
+
+internal static class PortAllocator
+{
+    public static int AllocateFreePort()
+    {
+        using var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        socket.Bind(new IPEndPoint(IPAddress.Loopback, 0));
+        return ((IPEndPoint)socket.LocalEndPoint!).Port;
+    }
+}