Speak telnet properly to the cluster node

The old client read the socket with a StreamReader, so the option negotiation a
node sends on connect landed in the text as control bytes and made a mess of
the first lines. TelnetStream now answers it: it agrees to ECHO and
SUPPRESS-GO-AHEAD, refuses everything else, swallows subnegotiations, and
tracks what it already answered so the two ends do not reply to each other for
ever. A command split across two reads is still understood, because a socket
hands over whatever has turned up rather than whole messages.

LineAssembler keeps the tail that has no line ending yet. Nodes write their
login prompt as "login: " with nothing after it, so the old ReadLineAsync sat
waiting for a line that never came and only got through because the greeting
happened to mention a matching word.

Login now follows N1MM: it looks for LOGON, ENTER CALL, LOG IN and the rest,
and sends the callsign anyway after ten seconds if no prompt turns up. A
password is sent when the node asks for one and the operator configured one.
The commands go out after that, not straight after the call. A connection that
has heard nothing for four minutes gets a blank line so the node does not drop
it as idle.

The old prompt check matched "call" anywhere in a line, which any spot comment
could trigger.

SendSpotAsync sends the node's dx command. Alt+P, or Edit / Spot It, spots the
call being typed at the current frequency, or the last contact logged when
nothing is typed, and puts it on our own bandmap without waiting for it to come
back round from the node.

The spot parser now takes a line with no colon after the spotter, and finds the
time when DXSpider has put the spotter's grid after it.

The cluster tests run over a real socket against a node fake that sends the
negotiation, the prompt and spot lines.

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

tests/Nonemm.Spotting.Tests/FakeTelnetStream.cs

@@ -0,0 +1,50 @@
+namespace Nonemm.Spotting.Tests;
+
+/// A stream that hands out bytes a test prepared and keeps whatever is written
+/// back, so the telnet negotiation can be checked without a socket. Each read
+/// returns one prepared block, the way a socket hands over one packet.
+public sealed class FakeTelnetStream : Stream
+{
+    private readonly Queue<byte[]> blocks;
+    private readonly List<byte> written = [];
+
+    public FakeTelnetStream(params byte[][] blocks) => this.blocks = new Queue<byte[]>(blocks);
+
+    public IReadOnlyList<byte> Written => written;
+
+    public override bool CanRead => true;
+
+    public override bool CanSeek => false;
+
+    public override bool CanWrite => true;
+
+    public override long Length => throw new NotSupportedException();
+
+    public override long Position
+    {
+        get => throw new NotSupportedException();
+        set => throw new NotSupportedException();
+    }
+
+    public override int Read(byte[] buffer, int offset, int count)
+    {
+        if (blocks.Count == 0)
+        {
+            return 0;
+        }
+        byte[] block = blocks.Dequeue();
+        block.CopyTo(buffer, offset);
+        return block.Length;
+    }
+
+    public override void Write(byte[] buffer, int offset, int count) =>
+        written.AddRange(buffer.Skip(offset).Take(count));
+
+    public override void Flush()
+    {
+    }
+
+    public override long Seek(long offset, SeekOrigin origin) => throw new NotSupportedException();
+
+    public override void SetLength(long value) => throw new NotSupportedException();
+}