feat: execute post-baseline jetstream parity plan

src/NATS.Server/LeafNodes/LeafLoopDetector.cs

@@ -0,0 +1,26 @@
+namespace NATS.Server.LeafNodes;
+
+public static class LeafLoopDetector
+{
+    private const string LeafLoopPrefix = "$LDS.";
+
+    public static string Mark(string subject, string serverId)
+        => $"{LeafLoopPrefix}{serverId}.{subject}";
+
+    public static bool IsLooped(string subject, string localServerId)
+        => subject.StartsWith($"{LeafLoopPrefix}{localServerId}.", StringComparison.Ordinal);
+
+    public static bool TryUnmark(string subject, out string unmarked)
+    {
+        unmarked = subject;
+        if (!subject.StartsWith(LeafLoopPrefix, StringComparison.Ordinal))
+            return false;
+
+        var serverSeparator = subject.IndexOf('.', LeafLoopPrefix.Length);
+        if (serverSeparator < 0 || serverSeparator == subject.Length - 1)
+            return false;
+
+        unmarked = subject[(serverSeparator + 1)..];
+        return true;
+    }
+}