feat(jetstream): add mirror sync loop and source coordination with filtering (C9+C10)

src/NATS.Server/JetStream/Storage/StoredMessage.cs

@@ -8,4 +8,14 @@ public sealed class StoredMessage
     public DateTime TimestampUtc { get; init; } = DateTime.UtcNow;
     public string? Account { get; init; }
     public bool Redelivered { get; init; }
+
+    /// <summary>
+    /// Optional message headers. Used for deduplication (Nats-Msg-Id) and source tracking.
+    /// </summary>
+    public IReadOnlyDictionary<string, string>? Headers { get; init; }
+
+    /// <summary>
+    /// Convenience accessor for the Nats-Msg-Id header value, used by source deduplication.
+    /// </summary>
+    public string? MsgId => Headers is not null && Headers.TryGetValue("Nats-Msg-Id", out var id) ? id : null;
 }