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.

tests/NATS.Server.Core.Tests/Subscriptions/SubListParityBatch2Tests.cs

@@ -121,6 +121,12 @@ public class SubListParityBatch2Tests
         {
         }
 
+        public void SendMessageNoFlush(string subject, string sid, string? replyTo, ReadOnlyMemory<byte> headers, ReadOnlyMemory<byte> payload)
+        {
+        }
+
+        public void SignalFlush() { }
+
         public bool QueueOutbound(ReadOnlyMemory<byte> data) => true;
 
         public void RemoveSubscription(string sid)