perf: optimize fan-out hot path and switch benchmarks to Release build

Round 9 optimizations targeting per-delivery overhead:
- Switch benchmark harness from Debug to Release build (biggest impact:
  durable fetch 0.42x→0.92x, request-reply to parity)
- Batch server-wide stats after fan-out loop (2 Interlocked per delivery → 2 per publish)
- Guard auto-unsub tracking with MaxMessages > 0 (skip Interlocked in common case)
- Cache SID as ASCII bytes on Subscription (avoid per-delivery encoding)
- Pre-encode subject bytes once before fan-out loop (avoid N encodings)
- Add 1-element subject string cache in ProcessPub (avoid repeated alloc)
- Remove Interlocked from SubList.Match stats counters (approximate is fine)
- Extract WriteMessageToBuffer helper for both string and span overloads

tests/NATS.Server.Benchmark.Tests/Infrastructure/DotNetServerProcess.cs

@@ -139,14 +139,14 @@ public sealed class DotNetServerProcess : IAsyncDisposable
         {
             if (File.Exists(Path.Combine(dir.FullName, "NatsDotNet.slnx")))
             {
-                var dll = Path.Combine(dir.FullName, "src", "NATS.Server.Host", "bin", "Debug", "net10.0", "NATS.Server.Host.dll");
+                var dll = Path.Combine(dir.FullName, "src", "NATS.Server.Host", "bin", "Release", "net10.0", "NATS.Server.Host.dll");
                 if (File.Exists(dll))
                     return dll;
 
                 var build = Process.Start(new ProcessStartInfo
                 {
                     FileName = "dotnet",
-                    Arguments = "build src/NATS.Server.Host/NATS.Server.Host.csproj -c Debug",
+                    Arguments = "build src/NATS.Server.Host/NATS.Server.Host.csproj -c Release",
                     WorkingDirectory = dir.FullName,
                     UseShellExecute = false,
                     RedirectStandardOutput = true,