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;