feat: implement SubscriptionIndex + JetStreamMemStore cluster — 39 features verified

Add SubscriptionIndex factory methods, notification wrappers, and
ValidateMapping. Implement 24 MemStore methods (TTL, scheduling, SDM,
age-check, purge/compact/reset) with JetStream header helpers and
constants. Verified features: 987 → 1026.

dotnet/src/ZB.MOM.NatsNet.Server/Internal/DataStructures/StreamDeletionMeta.cs

@@ -74,6 +74,21 @@ public sealed class StreamDeletionMeta
         return false;
     }
 
+    /// <summary>
+    /// Tries to get the pending entry for <paramref name="seq"/>.
+    /// </summary>
+    public bool TryGetPending(ulong seq, out SdmBySeq entry) => _pending.TryGetValue(seq, out entry);
+
+    /// <summary>
+    /// Sets the pending entry for <paramref name="seq"/>.
+    /// </summary>
+    public void SetPending(ulong seq, SdmBySeq entry) => _pending[seq] = entry;
+
+    /// <summary>
+    /// Returns the pending count for <paramref name="subj"/>, or 0 if not tracked.
+    /// </summary>
+    public ulong GetSubjectTotal(string subj) => _totals.TryGetValue(subj, out var cnt) ? cnt : 0;
+
     /// <summary>
     /// Clears all tracked data.
     /// Mirrors <c>SDMMeta.empty</c>.

dotnet/src/ZB.MOM.NatsNet.Server/Internal/DataStructures/SubscriptionIndex.cs

@@ -1096,6 +1096,14 @@ public sealed class SubscriptionIndex
         return false;
     }
 
+    // Write lock must be held.
+    private Exception? AddInsertNotify(string subject, Action<bool> notify)
+        => AddNotify(_notify!.Insert, subject, notify);
+
+    // Write lock must be held.
+    private Exception? AddRemoveNotify(string subject, Action<bool> notify)
+        => AddNotify(_notify!.Remove, subject, notify);
+
     private static Exception? AddNotify(Dictionary<string, List<Action<bool>>> m, string subject, Action<bool> notify)
     {
         if (m.TryGetValue(subject, out var chs))
@@ -1531,6 +1539,9 @@ public sealed class SubscriptionIndex
         public List<Subscription>? PList;
         public SublistLevel? Next;
 
+        /// <summary>Factory method matching Go's <c>newNode()</c>.</summary>
+        public static SublistNode NewNode() => new();
+
         public bool IsEmpty()
         {
             return PSubs.Count == 0 && (QSubs == null || QSubs.Count == 0) &&
@@ -1544,6 +1555,9 @@ public sealed class SubscriptionIndex
         public SublistNode? Pwc;
         public SublistNode? Fwc;
 
+        /// <summary>Factory method matching Go's <c>newLevel()</c>.</summary>
+        public static SublistLevel NewLevel() => new();
+
         public int NumNodes()
         {
             var num = Nodes.Count;

dotnet/src/ZB.MOM.NatsNet.Server/Internal/SubjectTransform.cs

@@ -243,6 +243,51 @@ public sealed class SubjectTransform : ISubjectTransformer
     public static (SubjectTransform? transform, Exception? err) NewStrict(string src, string dest) =>
         NewWithStrict(src, dest, true);
 
+    /// <summary>
+    /// Validates a subject mapping destination. Checks each token for valid syntax,
+    /// validates mustache-style mapping functions against known regexes, then verifies
+    /// the full transform can be created. Mirrors Go's <c>ValidateMapping</c>.
+    /// </summary>
+    public static Exception? ValidateMapping(string src, string dest)
+    {
+        if (string.IsNullOrEmpty(dest))
+            return null;
+
+        bool sfwc = false;
+        foreach (var t in dest.Split(SubjectTokens.Btsep))
+        {
+            var length = t.Length;
+            if (length == 0 || sfwc)
+                return new MappingDestinationException(t, ServerErrors.ErrInvalidMappingDestinationSubject);
+
+            // If it looks like a mapping function, validate against known patterns.
+            if (length > 4 && t[0] == '{' && t[1] == '{' && t[length - 2] == '}' && t[length - 1] == '}')
+            {
+                if (!PartitionRe.IsMatch(t) &&
+                    !WildcardRe.IsMatch(t) &&
+                    !SplitFromLeftRe.IsMatch(t) &&
+                    !SplitFromRightRe.IsMatch(t) &&
+                    !SliceFromLeftRe.IsMatch(t) &&
+                    !SliceFromRightRe.IsMatch(t) &&
+                    !SplitRe.IsMatch(t) &&
+                    !RandomRe.IsMatch(t))
+                {
+                    return new MappingDestinationException(t, ServerErrors.ErrUnknownMappingDestinationFunction);
+                }
+                continue;
+            }
+
+            if (length == 1 && t[0] == SubjectTokens.Fwc)
+                sfwc = true;
+            else if (t.AsSpan().ContainsAny("\t\n\f\r "))
+                return ServerErrors.ErrInvalidMappingDestinationSubject;
+        }
+
+        // Verify that the transform can actually be created.
+        var (_, err) = New(src, dest);
+        return err;
+    }
+
     /// <summary>
     /// Attempts to match a published subject against the source pattern.
     /// Returns the transformed subject or an error.