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/BenchmarkResult.cs

@@ -0,0 +1,27 @@
+namespace NATS.Server.Benchmark.Tests.Harness;
+
+/// <summary>
+/// Captures the results of a single benchmark run against one server.
+/// </summary>
+public sealed record BenchmarkResult
+{
+    public required string Name { get; init; }
+    public required string ServerType { get; init; }
+    public required long TotalMessages { get; init; }
+    public required long TotalBytes { get; init; }
+    public required TimeSpan Duration { get; init; }
+
+    /// <summary>Latency percentiles in microseconds, if measured.</summary>
+    public LatencyPercentiles? Latencies { get; init; }
+
+    public double MessagesPerSecond => TotalMessages / Duration.TotalSeconds;
+    public double BytesPerSecond => TotalBytes / Duration.TotalSeconds;
+    public double MegabytesPerSecond => BytesPerSecond / (1024.0 * 1024.0);
+}
+
+public sealed record LatencyPercentiles(
+    double P50Us,
+    double P95Us,
+    double P99Us,
+    double MinUs,
+    double MaxUs);