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>.