docs: refresh benchmark comparison with increased message counts

Increase message counts across all 14 benchmark test files to reduce
run-to-run variance (e.g. PubSub 16B: 10K→50K, FanOut: 10K→15K,
SinglePub: 100K→500K, JS tests: 5K→25K). Rewrite benchmarks_comparison.md
with fresh numbers from two-batch runs. Key changes: multi 4x4 reached
parity (1.01x), fan-out improved to 0.84x, TLS pub/sub shows 4.70x .NET
advantage, previous small-count anomalies corrected.

tests/NATS.Server.Benchmark.Tests/Mqtt/MqttThroughputTests.cs

@@ -15,7 +15,7 @@ public class MqttThroughputTests(MqttServerFixture fixture, ITestOutputHelper ou
     public async Task MqttPubSub_128B()
     {
         const int payloadSize = 128;
-        const int messageCount = 5_000;
+        const int messageCount = 25_000;
 
         var dotnetResult = await RunMqttPubSub("MQTT PubSub (128B)", "DotNet", fixture.DotNetMqttPort, payloadSize, messageCount);
 
@@ -35,7 +35,7 @@ public class MqttThroughputTests(MqttServerFixture fixture, ITestOutputHelper ou
     public async Task MqttCrossProtocol_NatsPub_MqttSub_128B()
     {
         const int payloadSize = 128;
-        const int messageCount = 5_000;
+        const int messageCount = 25_000;
 
         var dotnetResult = await RunCrossProtocol("Cross-Protocol NATS→MQTT (128B)", "DotNet", fixture.DotNetMqttPort, fixture.CreateDotNetNatsClient, payloadSize, messageCount);
 
@@ -55,7 +55,7 @@ public class MqttThroughputTests(MqttServerFixture fixture, ITestOutputHelper ou
         var payload = new byte[payloadSize];
         var topic = $"bench/mqtt/pubsub/{Guid.NewGuid():N}";
 
-        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(60));
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(120));
 
         var factory = new MqttFactory();
         using var subscriber = factory.CreateMqttClient();
@@ -127,7 +127,7 @@ public class MqttThroughputTests(MqttServerFixture fixture, ITestOutputHelper ou
         var natsSubject = $"bench.mqtt.cross.{Guid.NewGuid():N}";
         var mqttTopic = natsSubject.Replace('.', '/');
 
-        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(60));
+        using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(120));
 
         var factory = new MqttFactory();
         using var mqttSub = factory.CreateMqttClient();