feat(stream): add InterestRetentionPolicy for per-consumer ack tracking

Implements Interest retention policy logic that tracks which consumers
are interested in each message subject and whether they have acknowledged
delivery, retaining messages until all interested consumers have acked.
Go reference: stream.go checkInterestState/noInterest.

src/NATS.Server/JetStream/InterestRetentionPolicy.cs

@@ -0,0 +1,75 @@
+using NATS.Server.Subscriptions;
+
+namespace NATS.Server.JetStream;
+
+/// <summary>
+/// Tracks per-consumer interest and determines when messages can be removed
+/// under Interest retention policy. A message should be retained until all
+/// interested consumers have acknowledged it.
+/// Go reference: stream.go checkInterestState/noInterest.
+/// </summary>
+public sealed class InterestRetentionPolicy
+{
+    // consumer → filter subject pattern
+    private readonly Dictionary<string, string> _interests = new(StringComparer.Ordinal);
+    // seq → set of consumers that have acked this sequence
+    private readonly Dictionary<ulong, HashSet<string>> _acks = new();
+
+    /// <summary>
+    /// Register a consumer's interest in a subject pattern.
+    /// </summary>
+    public void RegisterInterest(string consumer, string filterSubject)
+    {
+        _interests[consumer] = filterSubject;
+    }
+
+    /// <summary>
+    /// Remove a consumer's interest (e.g., on deletion).
+    /// </summary>
+    public void UnregisterInterest(string consumer)
+    {
+        _interests.Remove(consumer);
+    }
+
+    /// <summary>
+    /// Record that a consumer has acknowledged delivery of a sequence.
+    /// </summary>
+    public void AcknowledgeDelivery(string consumer, ulong seq)
+    {
+        if (!_acks.TryGetValue(seq, out var ackedBy))
+        {
+            ackedBy = new HashSet<string>(StringComparer.Ordinal);
+            _acks[seq] = ackedBy;
+        }
+        ackedBy.Add(consumer);
+    }
+
+    /// <summary>
+    /// Returns true if the message should be retained (i.e., at least one
+    /// interested consumer has NOT yet acknowledged it).
+    /// A consumer is "interested" if its filter subject matches the message subject.
+    /// </summary>
+    public bool ShouldRetain(ulong seq, string msgSubject)
+    {
+        _acks.TryGetValue(seq, out var ackedBy);
+
+        foreach (var (consumer, filterSubject) in _interests)
+        {
+            // Check if this consumer is interested in this message's subject
+            if (!SubjectMatch.MatchLiteral(msgSubject, filterSubject))
+                continue;
+
+            // Consumer is interested — has it acked?
+            if (ackedBy == null || !ackedBy.Contains(consumer))
+                return true; // Not yet acked — must retain
+        }
+
+        // All interested consumers have acked (or no one is interested)
+        // Clean up ack tracking for this sequence
+        _acks.Remove(seq);
+        return false;
+    }
+
+    /// <summary>Number of registered consumers.</summary>
+    public int ConsumerCount => _interests.Count;
+}