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/jb_sim_test.go.txt

@@ -30,7 +30,7 @@ func sim(t *testing.T, name string, noiseFloor float64, ppm float64, jitterMs fu
 	for i := 0; i < 50; i++ { talkP = append(talkP, mk(i, true)); quietP = append(quietP, mk(i, false)) }
 	payloadFor := func(seq int) []byte { if seq%175 < 125 { return talkP[seq%50] }; return quietP[seq%50] }
 	dec, _ := opus.NewDecoder(rate, channels)
-	jb := newJitterBuffer(dec)
+	jb := newJitterBuffer(dec, 111)
 	r := rand.New(rand.NewSource(1))
 	const dur = 3 * 3600 * 1000.0
 	type ev struct{ at float64; seq uint16 }