perf: batch flush signaling and fetch path optimizations (Round 6)

Implement Go's pcd (per-client deferred flush) pattern to reduce write-loop
wakeups during fan-out delivery, optimize ack reply string construction with
stack-based formatting, cache CompiledFilter on ConsumerHandle, and pool
fetch message lists. Durable consumer fetch improves from 0.60x to 0.74x Go.

src/NATS.Server/JetStream/ConsumerManager.cs

@@ -375,6 +375,32 @@ public sealed class ConsumerManager : IDisposable
 
 public sealed record ConsumerHandle(string Stream, ConsumerConfig Config)
 {
+    /// <summary>
+    /// Compiled filter derived from Config. Cached per-instance; invalidated when Config
+    /// changes (either via <c>with { Config = newConfig }</c> or in-place mutation of
+    /// FilterSubject/FilterSubjects).
+    /// Go reference: consumer.go — filter subjects resolved once at consumer creation.
+    /// </summary>
+    private Consumers.CompiledFilter? _compiledFilter;
+    private string? _compiledFilterSubject;
+    private int _compiledFilterSubjectsCount;
+    public Consumers.CompiledFilter CompiledFilter
+    {
+        get
+        {
+            // Detect both reference change (with expression) and in-place mutation
+            if (_compiledFilter == null
+                || _compiledFilterSubject != Config.FilterSubject
+                || _compiledFilterSubjectsCount != Config.FilterSubjects.Count)
+            {
+                _compiledFilter = Consumers.CompiledFilter.FromConfig(Config);
+                _compiledFilterSubject = Config.FilterSubject;
+                _compiledFilterSubjectsCount = Config.FilterSubjects.Count;
+            }
+            return _compiledFilter;
+        }
+    }
+
     public ulong NextSequence { get; set; } = 1;
     public bool Paused { get; set; }
 

src/NATS.Server/JetStream/Consumers/PullConsumerEngine.cs

@@ -95,6 +95,9 @@ public sealed class CompiledFilter
 
 public sealed class PullConsumerEngine
 {
+    // Reusable fetch buffer to avoid per-fetch List allocation.
+    // Go reference: consumer.go — reuses slice backing array across fetch calls.
+    [ThreadStatic] private static List<StoredMessage>? t_fetchBuf;
     // Go: consumer.go — cluster-wide pending pull request tracking keyed by reply subject.
     // Reference: golang/nats-server/server/consumer.go waitingRequestsPending / proposeWaitingRequest
     private readonly ConcurrentDictionary<string, PullWaitingRequest> _clusterPending =
@@ -155,7 +158,11 @@ public sealed class PullConsumerEngine
     public async ValueTask<PullFetchBatch> FetchAsync(StreamHandle stream, ConsumerHandle consumer, PullFetchRequest request, CancellationToken ct)
     {
         var batch = Math.Max(request.Batch, 1);
-        var messages = new List<StoredMessage>(batch);
+        // Use thread-static buffer to avoid per-fetch List allocation.
+        // Results are snapshot'd into PullFetchBatch before the buffer is reused.
+        var messages = t_fetchBuf ??= new List<StoredMessage>();
+        messages.Clear();
+        if (messages.Capacity < batch) messages.Capacity = batch;
 
         // Go: consumer.go — enforce ExpiresMs timeout on pull fetch requests.
         // When ExpiresMs > 0, create a linked CancellationTokenSource that fires
@@ -215,8 +222,8 @@ public sealed class PullConsumerEngine
                 return new PullFetchBatch(messages);
         }
 
-        // Use CompiledFilter for efficient multi-filter matching
-        var compiledFilter = CompiledFilter.FromConfig(consumer.Config);
+        // Use cached CompiledFilter from ConsumerHandle (avoids per-fetch allocation)
+        var compiledFilter = consumer.CompiledFilter;
         var sequence = consumer.NextSequence;
 
         // Go: consumer.go — MaxBytes caps the total byte payload returned in one pull request
@@ -358,7 +365,8 @@ public sealed class PullFetchBatch
 
     public PullFetchBatch(IReadOnlyList<StoredMessage> messages, bool timedOut = false)
     {
-        Messages = messages;
+        // Snapshot: caller may reuse the list (ThreadStatic pooling), so take a copy.
+        Messages = messages.Count == 0 ? [] : messages.ToArray();
         TimedOut = timedOut;
     }
 }