Skip RTP padding packets instead of concealing them

Padding/probe packets use a sequence number but carry no audio. They were
treated as lost, so each one played an invented 20 ms frame (a glitch) and
pushed the buffer a frame deeper for drift correction to remove again.
Signature in the first live test: lost ~= drift-drops (74 vs 72 per minute).
They are now skipped and counted as 'padding'.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/pad_test.go.txt

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"fmt"
+	"math"
+	"testing"
+
+	"github.com/pion/rtp"
+	"gopkg.in/hraban/opus.v2"
+)
+
+func TestPadding(t *testing.T) {
+	for _, name := range []string{"empty payload", "other payload type"} {
+		enc, _ := opus.NewEncoder(rate, channels, opus.AppAudio)
+		dec, _ := opus.NewDecoder(rate, channels)
+		jb := newJitterBuffer(dec, 111)
+		pcm := make([]int16, frameSamps*channels)
+		buf := make([]byte, 4000)
+		seq, ts := uint16(0), 0.0
+		frames := 0
+		// 10 minutes of playout; audio packet every 20 ms, plus a padding packet every 40 seqs.
+		for tms := 0.0; tms < 600000; tms += 20 {
+			for ts <= tms {
+				if seq%40 == 39 {
+					p := &rtp.Packet{Header: rtp.Header{SequenceNumber: seq, PayloadType: 111}}
+					if name != "empty payload" {
+						p.PayloadType, p.Payload = 13, []byte{0x40}
+					}
+					jb.push(p)
+					seq++
+					continue
+				}
+				for k := range pcm { pcm[k] = int16(6000 * math.Sin(float64(frames*frameSamps+k/2)*2*math.Pi*220/rate)) }
+				n, _ := enc.Encode(pcm, buf)
+				jb.push(&rtp.Packet{Header: rtp.Header{SequenceNumber: seq, PayloadType: 111}, Payload: append([]byte(nil), buf[:n]...)})
+				seq++; frames++; ts += 20
+			}
+			jb.next()
+		}
+		c, d, tg := jb.snapshot()
+		fmt.Printf("%-20s padding=%d lost=%d fec=%d concealed=%d drift-drops=%d inserted=%d underruns=%d buffer=%d/%dms\n", name, c.Padding, c.Lost, c.FEC, c.PLC, c.Dropped, c.Inserted, c.Underruns, d, tg)
+		if c.Lost+c.PLC+c.Dropped+c.Underruns != 0 { t.Errorf("%s: padding still causes damage", name) }
+	}
+}