docs(workspace): explain host page and sidebar lifecycles

src/components/workspace/active-board.tsx

@@ -1,20 +1,28 @@
+/**
+ * 侧栏与画布页面共享的当前board元数据Context。只保存ID、工作区、路由引用与标题，
+ * 不保存元素、加载任务或ProjectEngine；目录改名可更新标题而无需重建业务画布。
+ */
 "use client";
 
 import { createContext, useContext, useMemo, useState, type Dispatch, type ReactNode, type SetStateAction } from "react";
 import type { Board } from "@/lib/workspace";
 
+/** null代表当前没有已解析画布，workspace_ref保留路径slug/引用与真实workspace_id的对应关系。 */
 type ActiveBoard = (Pick<Board, "id" | "workspace_id" | "title" | "slug"> & { workspace_ref?: string }) | null;
+/** 暴露React状态setter，调用方可条件更新以免迟到改名覆盖另一个活动board。 */
 const ActiveBoardContext = createContext<{
   board: ActiveBoard;
   setBoard: Dispatch<SetStateAction<ActiveBoard>>;
 } | null>(null);
 
+/** 在工作台布局内保持元数据，useMemo稳定Context对象，避免只因父渲染就通知全部使用方。 */
 export function ActiveBoardProvider({ children }: { children: ReactNode }) {
   const [board, setBoard] = useState<ActiveBoard>(null);
   const value = useMemo(() => ({ board, setBoard }), [board]);
   return <ActiveBoardContext.Provider value={value}>{children}</ActiveBoardContext.Provider>;
 }
 
+/** 缺少Provider时拒绝读取；不从浏览器路径凭空构造一条已认证board记录。 */
 export function useActiveBoard() {
   const context = useContext(ActiveBoardContext);
   if (!context) throw new Error("useActiveBoard requires ActiveBoardProvider");