feat: harden jetstream consumer state machine parity

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

@@ -3,9 +3,13 @@ namespace NATS.Server.JetStream.Consumers;
 public sealed class AckProcessor
 {
     private readonly Dictionary<ulong, PendingState> _pending = new();
+    public ulong AckFloor { get; private set; }
 
     public void Register(ulong sequence, int ackWaitMs)
     {
+        if (sequence <= AckFloor)
+            return;
+
         if (_pending.ContainsKey(sequence))
             return;
 
@@ -55,6 +59,9 @@ public sealed class AckProcessor
     {
         foreach (var key in _pending.Keys.Where(k => k <= sequence).ToArray())
             _pending.Remove(key);
+
+        if (sequence > AckFloor)
+            AckFloor = sequence;
     }
 
     private sealed class PendingState