合成流（program）

#define RTC_MCU_PUBLISHER_UID "__mcu__"   // 合成流发布者 uid（音视频共用此身份）
#define RTC_TRACK_AMCU_ID     "__amcu__"  // 音频合成流 trackId
#define RTC_TRACK_MCU_ID      "__mcu__"   // 视频合成流 trackId

int rtc_publish_mcu_video_track(void* handle, void* track_handle, rtc_publish_options_t* options);

int rtc_publish_mcu_audio_track(void* handle, void* track_handle, rtc_publish_options_t* options);

// 视频合成流
void* v = rtc_create_local_track(RTC_CODEC_H264);
rtc_publish_options_t vopts = {0};
strcpy(vopts.desc, "mcu_video");
vopts.width = 1280; vopts.height = 720; vopts.fps = 25;
rtc_publish_mcu_video_track(rtc, v, &vopts);      // trackId 固定为 __mcu__

// 音频混音流
void* a = rtc_create_local_track(RTC_CODEC_OPUS);
rtc_publish_options_t aopts = {0};
strcpy(aopts.desc, "mcu_audio");
aopts.sample_rate = 48000; aopts.channel_count = 2;
rtc_publish_mcu_audio_track(rtc, a, &aopts);      // trackId 固定为 __amcu__

// 推数据与普通轨道完全一致
rtc_write_sample(v, h264_frame, h264_len, 3600);  // 25fps: 90000/25
rtc_write_sample(a, opus_frame, opus_len, 960);

rtc_subscribe_video(rtc, RTC_MCU_PUBLISHER_UID, RTC_TRACK_MCU_ID);   // 订视频合成流
rtc_subscribe_audio(rtc, RTC_MCU_PUBLISHER_UID, RTC_TRACK_AMCU_ID);  // 订音频合成流

// 数据照常从 rtc_set_track_sample_callback 回调出来

// 取消订阅同样传保留常量
rtc_unsubscribe(rtc, RTC_MCU_PUBLISHER_UID, RTC_TRACK_MCU_ID);
rtc_unsubscribe(rtc, RTC_MCU_PUBLISHER_UID, RTC_TRACK_AMCU_ID);