docs(canvas): explain editor entry migration gates and loading lifecycles

src/lib/canvas-loader.ts

@@ -1,7 +1,13 @@
+/**
+ * 旧版画布的联合读取工具，并行取得目录元数据与v2快照。
+ * 它不协商canonical新模型，迁移感知的页面入口应使用canvas-entry中的分流流程。
+ */
 import { CANVAS_ACCEPT } from "./canvas-v2.ts";
 import { decodeSnapshot, type CanvasSnapshot, type CanvasRequest } from "./canvas-sync.ts";
 import { matchesRef, type Board } from "./workspace.ts";
 
+/** 逐段编码路由引用，校验board身份和工作区，再解码快照；AbortSignal传给两次请求。
+ * 返回数据不直接更新UI，导航是否仍有效由调用方控制。 */
 export async function loadBoardCanvas(request: CanvasRequest, workspaceRef: string, boardRef: string, signal?: AbortSignal) {
   const path = `workspaces/${encodeURIComponent(workspaceRef)}/materials/${encodeURIComponent(boardRef)}`;
   const [board, snapshot] = await Promise.all([request<Board>(path, { signal }), request<CanvasSnapshot>(path + "/canvas", { signal, headers: { Accept: CANVAS_ACCEPT } })]);