feat: add consumer reset to specific sequence (Gap 3.12)

Add ResetToSequence to ConsumerManager that updates NextSequence,
clears AckProcessor state via new ClearAll(), and zeroes PendingBytes.
Add AckProcessor.SetAckFloor() that prunes pending entries below the
new floor. Go reference: consumer.go:4241 processResetReq.

src/NATS.Server/JetStream/ConsumerManager.cs

@@ -225,6 +225,28 @@ public sealed class ConsumerManager : IDisposable
         return true;
     }
 
+    /// <summary>
+    /// Resets a consumer's position to the specified sequence.
+    /// Clears pending acks and redelivery state.
+    /// Go reference: consumer.go:4241 processResetReq.
+    /// </summary>
+    public bool ResetToSequence(string stream, string durableName, ulong sequence)
+    {
+        if (!_consumers.TryGetValue((stream, durableName), out var handle))
+            return false;
+
+        // Update the consumer's next sequence
+        handle.NextSequence = sequence;
+
+        // Clear pending acks — all outstanding acks are invalid after reset
+        handle.AckProcessor.ClearAll();
+
+        // Clear pending bytes
+        handle.PendingBytes = 0;
+
+        return true;
+    }
+
     public bool Unpin(string stream, string durableName)
     {
         return _consumers.ContainsKey((stream, durableName));

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

@@ -294,6 +294,32 @@ public sealed class AckProcessor
         _pending.Remove(sequence);
     }
 
+    /// <summary>
+    /// Clears all pending acks, terminated set, and resets the ack floor.
+    /// Used during consumer reset to specific sequence.
+    /// Go reference: consumer.go processResetReq — clear all tracking state.
+    /// </summary>
+    public void ClearAll()
+    {
+        _pending.Clear();
+        _terminated.Clear();
+        AckFloor = 0;
+        TerminatedCount = 0;
+        _exceededSequences.Clear();
+    }
+
+    /// <summary>
+    /// Resets the ack floor to the specified value.
+    /// Used during consumer reset.
+    /// </summary>
+    public void SetAckFloor(ulong floor)
+    {
+        AckFloor = floor;
+        // Remove any pending entries below or at the new floor
+        foreach (var key in _pending.Keys.Where(k => k <= floor).ToArray())
+            _pending.Remove(key);
+    }
+
     public bool HasPending => _pending.Count > 0;
     public int PendingCount => _pending.Count;