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/Harness/BenchmarkRunner.cs

@@ -0,0 +1,80 @@
+using System.Diagnostics;
+
+namespace NATS.Server.Benchmark.Tests.Harness;
+
+/// <summary>
+/// Lightweight benchmark runner with warmup + timed measurement.
+/// </summary>
+public sealed class BenchmarkRunner
+{
+    public int WarmupCount { get; init; } = 1_000;
+    public int MeasurementCount { get; init; } = 100_000;
+
+    /// <summary>
+    /// Measures throughput for a fire-and-forget style workload (pub-only or pub+sub).
+    /// The <paramref name="action"/> is called <see cref="MeasurementCount"/> times.
+    /// </summary>
+    public async Task<BenchmarkResult> MeasureThroughputAsync(
+        string name,
+        string serverType,
+        int payloadSize,
+        Func<int, Task> action)
+    {
+        // Warmup
+        for (var i = 0; i < WarmupCount; i++)
+            await action(i);
+
+        // Measurement
+        var sw = Stopwatch.StartNew();
+        for (var i = 0; i < MeasurementCount; i++)
+            await action(i);
+        sw.Stop();
+
+        return new BenchmarkResult
+        {
+            Name = name,
+            ServerType = serverType,
+            TotalMessages = MeasurementCount,
+            TotalBytes = (long)MeasurementCount * payloadSize,
+            Duration = sw.Elapsed,
+        };
+    }
+
+    /// <summary>
+    /// Measures latency for a request-reply style workload.
+    /// Records per-iteration round-trip time and computes percentiles.
+    /// </summary>
+    public async Task<BenchmarkResult> MeasureLatencyAsync(
+        string name,
+        string serverType,
+        int payloadSize,
+        Func<int, Task> roundTripAction)
+    {
+        // Warmup
+        for (var i = 0; i < WarmupCount; i++)
+            await roundTripAction(i);
+
+        // Measurement with per-iteration timing
+        var tracker = new LatencyTracker(MeasurementCount);
+        var overallSw = Stopwatch.StartNew();
+
+        for (var i = 0; i < MeasurementCount; i++)
+        {
+            var start = Stopwatch.GetTimestamp();
+            await roundTripAction(i);
+            tracker.Record(Stopwatch.GetTimestamp() - start);
+        }
+
+        overallSw.Stop();
+
+        return new BenchmarkResult
+        {
+            Name = name,
+            ServerType = serverType,
+            TotalMessages = MeasurementCount,
+            TotalBytes = (long)MeasurementCount * payloadSize,
+            Duration = overallSw.Elapsed,
+            Latencies = tracker.ComputePercentiles(),
+        };
+    }
+}