Add --tone-test mode: 440 Hz Opus tone + incoming audio capture

Adds a --tone-test flag that bypasses all OpenAL/soundcard
initialization and instead:

- Generates a 440 Hz sine wave at 48kHz mono, encodes it via Opus,
  and sends it to the Mumble server.

- Saves all incoming decoded audio to a raw PCM file (s16le,
  stereo, 48kHz) specified by --tone-out (default: incoming.pcm).

Useful for end-to-end testing of the Opus encode/decode pipeline
and Mumble transport without requiring physical audio hardware.

Usage:
  barnard --server HOST --tone-test --tone-out /tmp/in.pcm

  ffplay -f s16le -ar 48000 -ac 2 incoming.pcm

barnard.go

@@ -63,6 +63,12 @@ type Barnard struct {
 	FileStream      *fileplayback.Player
 	FileStreamMutex sync.Mutex
 
+	// Added for tone test mode (bypasses all soundcard/OpenAL)
+	ToneTest       bool
+	ToneTestOutput string
+	toneTestStop   chan struct{}
+	toneTestSaver  *AudioFileSaver
+
 	// Added for recording
 	RecordingMutex    sync.Mutex
 	Recorder          *recording.Recorder
@@ -80,7 +86,15 @@ func (b *Barnard) StopTransmission() {
 		b.Notify("micdown", "me", "")
 		b.Tx = false
 		b.UpdateGeneralStatus(" Idle ", false)
-		b.Stream.StopSource()
+		if b.ToneTest {
+			// Stop the tone generator.
+			if b.toneTestStop != nil {
+				close(b.toneTestStop)
+				b.toneTestStop = nil
+			}
+		} else if b.Stream != nil {
+			b.Stream.StopSource()
+		}
 	}
 }