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>

tests/Nonemm.Spotting.Tests/TelnetStreamTests.cs

@@ -38,8 +38,10 @@ public class TelnetStreamTests
         Assert.Equal("DX de W3LPL: 14025.0 JA1XYZ\r\n", read);
     }
 
-    /// A client that leaves the negotiation unanswered gets these bytes mixed
-    /// into the first line it reads.
+    /// N1MM never answers the negotiation. It drops bytes 251, 252 and 255 from
+    /// what it displays but lets 253 and 254 through, so a node that opens with
+    /// IAC DO TERMINAL-TYPE puts two stray characters in its packet window and
+    /// leaves the node waiting for an answer that never comes. We answer.
     [Fact]
     public async Task TheNegotiationIsAnsweredAndKeptOutOfTheText()
     {