reuse the stereo Opus encoder across connections

Every connect built a fresh stereo encoder for file playback. Each one holds
a little under a megabyte of encoder state, so on a flaky link that is close
to a megabyte of churn per reconnect for no benefit; the encoder is already
reset when file playback ends.

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

barnard.go

@@ -70,6 +70,10 @@ type Barnard struct {
 	// Added for file playback
 	FileStream      *fileplayback.Player
 	FileStreamMutex sync.Mutex
+	// stereoEncoder is reused across connections. Each one holds a little
+	// under a megabyte of encoder state, so building a fresh one per
+	// reconnect is pure churn; it is reset when file playback ends.
+	stereoEncoder gumble.AudioEncoder
 
 	// Added for tone test mode (bypasses all soundcard/OpenAL)
 	ToneTest            bool