feat: implement strict retention runtime parity for jetstream

src/NATS.Server/JetStream/ConsumerManager.cs

@@ -12,6 +12,7 @@ public sealed class ConsumerManager
 {
     private readonly JetStreamMetaGroup? _metaGroup;
     private readonly ConcurrentDictionary<(string Stream, string Name), ConsumerHandle> _consumers = new();
+    private readonly ConcurrentDictionary<string, ulong> _ackFloors = new(StringComparer.Ordinal);
     private readonly PullConsumerEngine _pullConsumerEngine = new();
     private readonly PushConsumerEngine _pushConsumerEngine = new();
 
@@ -130,6 +131,7 @@ public sealed class ConsumerManager
             return false;
 
         handle.AckProcessor.AckAll(sequence);
+        _ackFloors.AddOrUpdate(stream, _ => sequence, (_, existing) => Math.Max(existing, sequence));
         return true;
     }
 
@@ -180,6 +182,9 @@ public sealed class ConsumerManager
 
         return true;
     }
+
+    internal ulong GetAckFloor(string stream)
+        => _ackFloors.TryGetValue(stream, out var ackFloor) ? ackFloor : 0;
 }
 
 public sealed record ConsumerHandle(string Stream, ConsumerConfig Config)