fix: add jitter buffer, sequence tracking, and mic error reporting

Add a jitter buffer in OnAudioStream that collects incoming audio packets,
sorts by sequence number, and releases them in order after a small initial
delay (3 packets, ~30ms). This prevents out-of-order playback from network
jitter and reordered UDP packets.

Add a Sequence field to AudioPacket so the jitter buffer can reorder by
sequence number. PLC frames generated by dispatchPLC now also carry the
correct sequence number so they integrate with the jitter buffer.

Add microphone capture error reporting: when CaptureSamples returns fewer
bytes than expected, report the failure to the user via a callback. The
error is reported once when the failure starts and again when the mic
recovers. Wire this up in client.go to display status messages.

client.go

@@ -53,6 +53,13 @@ func (b *Barnard) connect(reconnect bool) bool {
 	b.Stream = stream
 	b.Stream.AttachStream(b.Client)
 	b.Stream.SetNoiseProcessor(b.NoiseSuppressor)
+	b.Stream.SetErrorFunc(func(err error) {
+		if err != nil {
+			b.AddOutputLine(fmt.Sprintf("Microphone: %s", err.Error()))
+		} else {
+			b.AddOutputLine("Microphone: recovered")
+		}
+	})
 
 	// Initialize stereo encoder for file playback
 	b.Client.AudioEncoderStereo = opus.NewStereoEncoder()