桌面原生数据面：pion 数据通道取代 WebView WebRTC，逃离 256KB rwnd 与渲染器节流

把桌面 P2P 数据面从 WebView 内 JS WebRTC 移到 Go 进程的 pion，逃离 WebKit/WebView2 写死的 256KB SCTP 接收窗与隐藏窗口的渲染器节流。后端零改动、与现有客户端互通（线协议逐字节对齐）。真机实测原生↔原生 / 原生↔iOS（经有线）稳定 7-10MB/s，对比旧 JS 路径的 1.5MB/s 还塌缩是质变。设计与阶段见 desktop/NATIVE-TRANSFER.md。

- engine 包（纯 Go，仅 pion+stdlib、零 Wails 依赖，供将来 gomobile 复用于 iOS）：pion 收发，逐字节对齐 web 引擎线协议（DataChannel cdrop-file ordered、文本帧 meta/done/ack、二进制分片、信令 {type,sdp?,candidate?}）；SetSCTPMaxReceiveBufferSize 顶到 4MB；mDNS QueryOnly 解析对端 .local 候选实现同内网 host↔host 直连；接收端直接写盘（去 base64 桥 / OPFS）；进程内端到端测试 + 诊断日志（候选/ICE 态/选中对/发送停滞）。
- app.go：Bind P2PStartOutgoing / StartIncoming / HandleSignal / Cancel + 原生取文件 PickFileForSend + 中继回退读盘片 ReadFileSlice；引擎回调经 EventsEmit p2p:* 反向通知（进度/状态/信令/落盘路径/候选对）。
- JS p2pBackend 抽象（p2pNative.ts）：桌面按对端把会话委派给 Go 引擎，transfer.ts / hub.ts 零改动；HTTP 全留 JS（出站信令 POST、状态机 /p2p·接收端 /done·/fail）；监听 p2p:* 事件驱动 store、phase 与 iceStats。
- 发送取文件改原生路径：selectedFile 模型 File→FileSource（桌面带绝对 path → Go 直接读盘走原生；浏览器拖放无 path → 回退 JS）。Composer dropzone 在桌面打开原生文件对话框。
- 完成语义修复：接收端完成不立即拆连接（续发最终 ack、留 SSE 终态 Cancel 拆、60s 兜底、终态不可被后续 close 翻转）——修「iOS→mac 实际成功却被误标失败」。
- 剪贴板暂存路径修复：富文本经 Universal Clipboard 暂存为 .rtfd，pasteboard 纯文本表示有时取到该暂存路径——uploadClipboard 共享守卫拦截 + mac 原生读改从 RTF / RTFD 派生纯文本。

desktop/go.sum

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