Let MMTTY hold the text waiting to go out, behind a setting

MMTTY has a type-ahead buffer of its own: characters go into it, a backspace
takes back one it has not transmitted, and TxBufLen says how many are left. Used
that way it paces itself, so there is no gap between characters to tune and no
baud rate to keep in step with the engine.

EngineTypeAhead does that, and Config > Digital picks between it and the pump
that is there now. Off is still the default: three things it rests on have never
been seen with a real engine.

tools/Nonemm.EngineProbe asks the engine those three questions and writes the
answers to a file. It starts MMTTY through the bridge, pushes a message, polls
TxBufLen while it goes out, backspaces over text that has and has not been
transmitted, and logs what came back on the receive side and when.

The bridge learns one verb for it: `buffer` reads TxBufLen and answers with the
count, or -1 when the control will not say. A property the control does not know
is a log line rather than an error, since it stops nothing.

TypeAhead and EngineTypeAhead share the TransmitBuffer interface, which is what
the digital window now works through, so the window does not know which one it
has.

docs/unfinished.md states what each buffer assumes and how to run the probe.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RtspmWmS7f8kUvcyaHpRWZ

tools/Nonemm.EngineProbe/ProbeLog.cs

@@ -0,0 +1,45 @@
+namespace Nonemm.EngineProbe;
+
+/// The probe's report: every line on the screen and in the file, stamped with
+/// the milliseconds since the probe started. The timing is the point of the
+/// report, so nothing is written without it.
+public sealed class ProbeLog : IDisposable
+{
+    private readonly StreamWriter file;
+    private readonly Lock gate = new();
+    private readonly long started = Environment.TickCount64;
+
+    public ProbeLog(string path)
+    {
+        Path = System.IO.Path.GetFullPath(path);
+        file = new StreamWriter(Path, append: false) { AutoFlush = true };
+    }
+
+    public string Path { get; }
+
+    public long Elapsed => Environment.TickCount64 - started;
+
+    public void Write(string text)
+    {
+        lock (gate)
+        {
+            string line = $"{Elapsed,7} ms  {text}";
+            Console.WriteLine(line);
+            file.WriteLine(line);
+        }
+    }
+
+    /// A heading, so the file can be read a step at a time.
+    public void Step(string name)
+    {
+        lock (gate)
+        {
+            Console.WriteLine();
+            Console.WriteLine($"== {name}");
+            file.WriteLine();
+            file.WriteLine($"== {name}");
+        }
+    }
+
+    public void Dispose() => file.Dispose();
+}