dsh-ui-container: DeepSeek Harness UI Container and Remote Projection Plugin

`dsh-ui-container` is a browser extension based on DeepSeek Harness (DSH), designed to address the challenges of boundary control in frontend collaboration between extensions. It enables cross-frontend capability sharing by providing reusable UI surface containers and a lightweight remote protocol. The core mechanism involves transmitting JSON-compatible document snapshots and change notifications, rather than directly transferring React elements, DOM nodes, or Cordis service objects. The extension supports `MessagePort` cross-process channels and WebSocket network channels, utilizes the JSON-RPC 2.0 protocol, and negotiates capabilities such as document projection, subscriptions, and surface commands during the handshake phase. Applicable scenarios include reuse within Workers, iframe bridges, or independent frontends.