MediaStreamTrack applyConstraints：分辨率与帧率动态调整

概述`track.applyConstraints()` 允许在采集会话中调整参数。结合能力探测与网络状况选择合适的 `frameRate/width/height` 与 `advanced` 选项。用法/示例const stream = await navigator.mediaDevices.getUserMedia({ video: true }) const track = stream.getVideoTracks()[0] await track.applyConstraints({ width: 1280, height: 720, frameRate: 30 }) // 降级示例 await track.applyConstraints({ advanced: [{ frameRate: 24 }, { width: 640, height: 360 }] }) 工程建议与 `MediaCapabilities.encodingInfo` 协作选择平滑且省电的配置。避免频繁切换造成中断；记录失败并提供回退策略。在设备不支持的约束上捕获异常并提示用户。参考与验证MDN：MediaStreamTrack — https://developer.mozilla.org/docs/Web/API/MediaStreamTrackweb.dev：Constraints — https://web.dev/articles/media-constraints