fix(stream): no copiar AAC multicanal en modo copy (WebKit lo rechaza igual)

El downmix estéreo del re-encode (f89396c) dejaba un agujero simétrico: una
fuente cuyo audio YA es AAC 5.1 se copiaba tal cual, y WebKit rechaza el
AAC multicanal en el primer segmento exactamente igual que el re-encodeado.
Copy de audio ahora solo cuando la pista es AAC con ≤2 canales; cualquier
otra cosa (no-AAC, AAC 5.1+, o canales desconocidos en el probe — fail-safe)
re-encodea a AAC estéreo 48k. La pista multicanal original queda intacta
para reproductor externo. Test smoke nuevo: fuente AAC 5.1 → re-encode.

internal/engine/hls.go

@@ -1959,20 +1959,22 @@ func buildHLSCopyArgs(cfg HLSSessionConfig, probe *StreamProbe, tmpDir string) [
 		args = append(args, "-tag:v", "hvc1")
 	}
 
-	// Audio: copy when the SELECTED track is already AAC, else AAC 192k.
+	// Audio: copy ONLY when the selected track is AAC with ≤2 channels —
-	// (fMP4 HLS carries AAC universally; EAC3/DTS/TrueHD do not.)
+	// WebKit/Apple HLS rejects multichannel AAC at the first media segment
+	// (observed via the Safari access log: master → index → init → seg-0
+	// fetched twice, then silence — every 5.1 movie failed on iPhone while
+	// stereo-AAC episodes played). Anything else (non-AAC, or AAC 5.1+) is
+	// re-encoded mirroring the encode path exactly: AAC stereo 48k. The
+	// original multichannel track stays intact for external players.
 	audioCodec := probe.AudioCodec
+	audioChannels := 0
 	if audioIdx < len(probe.AudioTracks) {
 		audioCodec = probe.AudioTracks[audioIdx].Codec
+		audioChannels = probe.AudioTracks[audioIdx].Channels
 	}
-	if strings.EqualFold(audioCodec, "aac") {
+	if strings.EqualFold(audioCodec, "aac") && audioChannels > 0 && audioChannels <= 2 {
 		args = append(args, "-c:a", "copy")
 	} else {
-		// Mirror the encode path exactly: AAC stereo 48k. WITHOUT -ac 2 a 5.1
-		// source produces 6-channel ffmpeg-native AAC, which WebKit/Apple HLS
-		// rejects at the first media segment (observed via Safari access log:
-		// master → index → init → seg-0 fetched twice, then silence — every
-		// 5.1 movie failed on iPhone while stereo-AAC episodes played).
 		args = append(args, "-c:a", "aac", "-b:a", "192k", "-ar", "48000", "-ac", "2")
 	}
 

internal/engine/hls_copy_smoke_test.go

@@ -205,6 +205,25 @@ func TestHLSCopy_Hevc10Eac3_IncidentShape(t *testing.T) {
 	}
 }
 
+func TestHLSCopy_Aac51MustReencode(t *testing.T) {
+	// AAC is NOT copy-safe when multichannel: WebKit rejects 6-channel AAC at
+	// the first media segment exactly like re-encoded 5.1. Source AAC 5.1 →
+	// must re-encode to stereo, never copy.
+	rt := copyTestRuntime(t)
+	src := genSource(t, rt, "aac51.mkv",
+		[]string{"-c:v", "libx264", "-preset", "ultrafast", "-pix_fmt", "yuv420p"},
+		[]string{"-c:a", "aac", "-ac", "6", "-b:a", "256k"}, 8)
+	s, pl := runCopySession(t, rt, src, 0)
+	assertCopyOutput(t, rt, s, pl, "h264", "aac", 8)
+	args := buildHLSCopyArgs(s.cfg, s.probe, s.tmpDir)
+	if containsSeq(args, "-c:a", "copy") {
+		t.Errorf("AAC 5.1 must NOT be copied (WebKit rejects multichannel AAC), args: %v", args)
+	}
+	if !containsSeq(args, "-ac", "2") {
+		t.Errorf("AAC 5.1 must re-encode to stereo, args: %v", args)
+	}
+}
+
 func TestHLSCopy_ResumeStartSec(t *testing.T) {
 	rt := copyTestRuntime(t)
 	src := genSource(t, rt, "resume.mkv",