Take every out-of-band directive out of the connect screen, not just MCP

TelnetNegotiationCore's framing now follows the specification rather than
a stricter reading of it: a line-initial #$# that cannot be acted on is
dropped rather than passed through, in a session or outside one. MCP says
so plainly -- an unrecognised message on that prefix is silently dropped,
not shown to a reader.

For the crawler that means the other three lines go too. All 57
line-initial #$# lines in the catalogue now come out of the screen, not
just the 54 MCP offers: also two "#$# SDWC-*-NOWRAP" and one
"#$#LOGIN_TRIGGER", which are somebody's client directives sent on the
prefix reserved for exactly that, and which no reader should have been
shown either.

Line-initial is still what keeps ASCII art safe, and the catalogue is the
evidence: of the 59 screens matching #$# anywhere, the 3 that match only
mid-line are all art. Both cases have a test.

555 tests green.

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

Author: HarryCordewener

src/MUI.Crawl/Telnet/TelnetProbe.cs

@@ -1102,10 +1102,13 @@ private TelnetInterpreterBuilder Build(Observations seen, List<byte[]> lines, Pr
             //
             // Deliberately WITHOUT answering. The crawler has no use for an MCP session, and answering
             // would put text on a stranger's login prompt for one it will never open -- the objection
-            // MSSPPlaintextProtocol makes to sending MSSP-REQUEST unbidden. The offer is consumed
-            // either way; only the reply is suppressed. Nothing after the offer is stripped, because
-            // no session opens and the library treats a #$# line outside a session as ordinary output
-            // -- which is the conservative reading, and the one that keeps ASCII art safe.
+            // MSSPPlaintextProtocol makes to sending MSSP-REQUEST unbidden. Only the reply is
+            // suppressed: the framing is not conditional on a session, so every line-initial #$# comes
+            // out, which takes the other three with it -- two "#$# SDWC-*-NOWRAP" and one
+            // "#$#LOGIN_TRIGGER", somebody's client directives on the prefix reserved for exactly that.
+            //
+            // Line-initial is what keeps ASCII art safe, and the catalogue is the evidence: of the 59
+            // screens matching #$# anywhere, the 3 that match only mid-line are all art.
             //
             // Noted on the offer rather than on negotiation, because there is no negotiation: the
             // offer arriving is the whole of the evidence that this server speaks MCP.

tests/MUI.Crawl.Tests/Telnet/ProbeSessionTests.cs

@@ -1713,6 +1713,33 @@ public async Task TheCrawlerDoesNotAnswerAnMcpOffer()
         await Assert.That(game.Received.Any(line => line.StartsWith("#$#"))).IsFalse();
     }
 
+    /// <summary>
+    /// The other line-initial <c>#$#</c> lines in the catalogue go too, not just the MCP offer.
+    /// </summary>
+    /// <remarks>
+    /// <c>#$#</c> is the out-of-band prefix, and MCP says an unrecognised message on it is dropped
+    /// rather than shown. The three remaining line-initial matches in the catalogue -- two
+    /// <c>#$# SDWC-*-NOWRAP</c> and one <c>#$#LOGIN_TRIGGER</c> -- are somebody's client directives,
+    /// which is exactly what that prefix is for and exactly what a reader should not be shown.
+    /// </remarks>
+    [Test]
+    public async Task OtherOutOfBandDirectivesGoToo()
+    {
+        await using var game = new FakeGame
+        {
+            Banner = "#$# SDWC-START-NOWRAP\r\n"
+                + "Welcome to the game.\r\n"
+                + "#$#LOGIN_TRIGGER\r\n"
+                + "#$# SDWC-END-NOWRAP\r\n",
+            WhoReply = "0 Players logged in.\r\n",
+        };
+
+        var result = await new TelnetProbe(Fast()).ProbeAsync(game.Target);
+
+        await Assert.That(result.Banner).DoesNotContain("#$#");
+        await Assert.That(result.Banner).Contains("Welcome to the game.");
+    }
+
     /// <summary>
     /// <c>#$#</c> inside ASCII art is art, not protocol. Three of the 59 screens that match
     /// <c>#$#</c> anywhere match only in the middle of a line -- <c>d########   #$#</c>,