feat: enforce jetstream ack and redelivery semantics

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

@@ -0,0 +1,24 @@
+namespace NATS.Server.JetStream.Consumers;
+
+public sealed class AckProcessor
+{
+    private readonly Dictionary<ulong, DateTime> _pending = new();
+
+    public void Register(ulong sequence, int ackWaitMs)
+    {
+        _pending[sequence] = DateTime.UtcNow.AddMilliseconds(Math.Max(ackWaitMs, 1));
+    }
+
+    public ulong? NextExpired()
+    {
+        foreach (var (seq, deadline) in _pending)
+        {
+            if (DateTime.UtcNow >= deadline)
+                return seq;
+        }
+
+        return null;
+    }
+
+    public bool HasPending => _pending.Count > 0;
+}