gameserver: skip encryption on Hello (try plaintext challenge)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/Prospect.Unreal/Runtime/UWorld.cs

@@ -286,7 +286,16 @@ public abstract partial class UWorld : FNetworkNotify, IAsyncDisposable
                         }
                         else
                         {
-                            throw new NotImplementedException("Encryption");
+                            // The Cycle client offers an EncryptionToken in NMT_Hello. A UE
+                            // connection only switches to AES once the server replies with
+                            // NMT_EncryptionAck — the token is an offer, not a requirement.
+                            // Implementing the full AES-GCM packet handler is a large effort,
+                            // so first proceed WITHOUT encryption: never send the ack and go
+                            // straight to the challenge. If the client keeps talking plaintext
+                            // we're done; if it insists on encryption it will drop here and we
+                            // know AES is mandatory.
+                            Logger.Warning("Client sent EncryptionToken ({Len} chars) — proceeding WITHOUT encryption (no EncryptionAck)", encryptionToken.Length);
+                            connection.SendChallengeControlMessage();
                         }
                     }
                     break;