Break cluster lines on a bare carriage return

N1MM splits the telnet stream on CR, not LF, which says some nodes send CR with
no LF after it. Against those the assembler was handing the whole session back
as one unfinished line.

Also adds "user:" to the login prompts, which N1MM matches and we did not.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/Nonemm.Spotting/Telnet/ClusterPrompts.cs

@@ -7,7 +7,7 @@ public static class ClusterPrompts
     private static readonly string[] Callsign =
     [
         "login", "log in", "logon", "enter call", "enter your call",
-        "your call", "callsign", "call:",
+        "your call", "callsign", "call:", "user:",
     ];
 
     private static readonly string[] Password = ["password"];