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

src/Nonemm.Digital/MmttyEngine.cs

@@ -54,6 +54,10 @@ public sealed class MmttyEngine : DigitalEngine
     /// Anything the bridge says about itself, for the status line.
     public event EventHandler<string>? Reported;
 
+    /// The answer to `AskBufferedAsync`: how many characters the engine still
+    /// has to transmit.
+    public event EventHandler<int>? Buffered;
+
     public async Task StartAsync(CancellationToken cancellation = default)
     {
         settings = MmttySettings.Read(options.SettingsPath);
@@ -75,6 +79,13 @@ public sealed class MmttyEngine : DigitalEngine
     public Task TypeAsync(char character, CancellationToken cancellation = default) =>
         PostAsync(MmttyMessage.TypeCharacter, character, cancellation);
 
+    /// Asks how many characters the engine still holds. The control has no
+    /// event for it, so it is polled; the answer arrives on `Buffered`. A
+    /// property name other than `TxBufLen` is only for finding out what the
+    /// control answers to.
+    public Task AskBufferedAsync(string property = "", CancellationToken cancellation = default) =>
+        bridge.SendAsync(BridgeLine.Write("buffer", property), cancellation);
+
     /// N1MM's abort, which is what its RX button and Escape do: drop PTT and
     /// let the engine stop where it is.
     public Task AbortAsync(CancellationToken cancellation = default) =>
@@ -154,6 +165,9 @@ public sealed class MmttyEngine : DigitalEngine
             case "rx":
                 Received?.Invoke(this, ((char)Number(fields, 0)).ToString());
                 break;
+            case "buffer":
+                Buffered?.Invoke(this, Number(fields, 0));
+                break;
             case "tx":
                 IsTransmitting = Number(fields, 0) == 1;
                 TransmitChanged?.Invoke(this, IsTransmitting);