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

src/NATS.Server/Subscriptions/SubList.cs

@@ -554,7 +554,7 @@ public sealed class SubList : IDisposable
 
     public SubListResult Match(string subject)
     {
-        Interlocked.Increment(ref _matches);
+        _matches++;
         var currentGen = Interlocked.Read(ref _generation);
 
         _lock.EnterReadLock();
@@ -562,7 +562,7 @@ public sealed class SubList : IDisposable
         {
             if (_cache != null && _cache.TryGetValue(subject, out var cached) && cached.Generation == currentGen)
             {
-                Interlocked.Increment(ref _cacheHits);
+                _cacheHits++;
                 return cached.Result;
             }
         }
@@ -581,7 +581,7 @@ public sealed class SubList : IDisposable
             currentGen = Interlocked.Read(ref _generation);
             if (_cache != null && _cache.TryGetValue(subject, out var cached) && cached.Generation == currentGen)
             {
-                Interlocked.Increment(ref _cacheHits);
+                _cacheHits++;
                 return cached.Result;
             }
 
@@ -940,8 +940,8 @@ public sealed class SubList : IDisposable
             _lock.ExitReadLock();
         }
 
-        var matches = Interlocked.Read(ref _matches);
-        var cacheHits = Interlocked.Read(ref _cacheHits);
+        var matches = Volatile.Read(ref _matches);
+        var cacheHits = Volatile.Read(ref _cacheHits);
         var hitRate = matches > 0 ? (double)cacheHits / matches : 0.0;
 
         uint maxFanout = 0;

src/NATS.Server/Subscriptions/Subscription.cs

@@ -1,3 +1,4 @@
+using System.Text;
 using NATS.Server;
 using NATS.Server.Imports;
 
@@ -5,9 +6,17 @@ namespace NATS.Server.Subscriptions;
 
 public sealed class Subscription
 {
+    private byte[]? _sidBytes;
+
     public required string Subject { get; init; }
     public string? Queue { get; init; }
     public required string Sid { get; init; }
+
+    /// <summary>
+    /// Pre-encoded ASCII bytes of the SID, cached to avoid per-delivery encoding.
+    /// </summary>
+    public byte[] SidBytes => _sidBytes ??= Encoding.ASCII.GetBytes(Sid);
+
     public long MessageCount;  // Interlocked
     public long MaxMessages;   // 0 = unlimited
     public INatsClient? Client { get; set; }