渐进式生成播放：fMP4 前 4 个分片就绪即返回播放列表开始起播，后台边转边补 ENDLIST，消除整集封装等待的黑屏缓冲

internal/app/playback_plan_test.go

@@ -215,7 +215,7 @@ func TestPlaybackGeneratedPreservesLongGOPAndAudioOnlyVideo(t *testing.T) {
 			}
 			ctx, stop := context.WithTimeout(parent, 30*time.Second)
 			defer stop()
-			if err := app.generatePlayback(ctx, media, processing, "/media/", ""); err != nil {
+			if err := app.generatePlayback(ctx, media, processing, "/media/", "", 0); err != nil {
 				media.Close()
 				t.Fatal(err)
 			}
@@ -224,6 +224,13 @@ func TestPlaybackGeneratedPreservesLongGOPAndAudioOnlyVideo(t *testing.T) {
 			if media.generated.ctx.Err() != nil {
 				t.Fatal("completed media inherited its preparation timeout")
 			}
+			completeDeadline := time.Now().Add(20 * time.Second)
+			for !media.generated.complete.Load() {
+				if time.Now().After(completeDeadline) {
+					t.Fatal("background remux did not complete")
+				}
+				time.Sleep(20 * time.Millisecond)
+			}
 			parts := media.generated.segments
 			if len(parts) != 4 || parts[0].Duration < 2.99 || parts[0].Duration > 3.01 || parts[1].Start != parts[0].Duration || !strings.Contains(media.generated.playlist, "#EXT-X-TARGETDURATION:3") {
 				t.Fatalf("invented fixed two-second timeline: %+v", parts)