多频道

let main = try await srtc.joinChannel(token: mainToken)
let group = try await srtc.joinChannel(token: groupToken)

srtc.channels          // [main, group]，按加入先后排序
srtc.defaultChannel    // main（最早加入且仍存活的那个）

await group.leave()            // 或 srtc.leaveChannel(group)
await srtc.leaveChannel()      // 无参版本作用于 defaultChannel

let camera = srtc.createLocalCameraTrack(preset: .h720p)
try await camera.startCapture()

try await main.publishLocalTrack(camera)
try await group.publishLocalTrack(camera)   // 同一路采集，两个频道各自发送

await group.leave()      // camera 仍被 main 发布 → 保持采集
await main.leave()        // 最后一个持有者 → 自动停止采集

let mic = srtc.createLocalMicTrack()
try await mic.startCapture()

try await main.publishLocalTrack(mic)
try await group.publishLocalTrack(mic)     // ✅ 两个频道的音频源集合都是 {mic}

try await main.publishLocalTrack(mic)                    // {mic}
try await group.publishLocalTrack(mic)                   // {mic}
try await group.publishLocalTrack(screenTrack)           // ❌ group 变成 {mic, screen_audio}
// SRTCError.invalidState：各频道发布的音频源必须完全相同

main.delegates.add(delegate: mainHandler)
group.delegates.add(delegate: groupHandler)