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; }