perf(canvas): 大画布渲染与媒体 - 布局不变时复用空间索引，合并重复的资源与封面请求

连线索引只存端点 ID，文本更新不再重建几何。小地图一次遍历隐藏折叠帧子节点。资源 Blob 与签名刷新合并在途请求，视频封面优先复用已缓存文件。

web/src/services/api/resources.ts

@@ -387,7 +387,8 @@ export async function getResourceAccess(storageKey: string | undefined, purpose:
     const forceRefresh = fetchOptions.forceRefresh === true;
     if (forceRefresh) {
         accessCache.delete(key);
-        accessRequests.delete(key);
+        const inFlightRefresh = accessRequests.get(key);
+        if (inFlightRefresh) return inFlightRefresh;
         if (purpose === "display") {
             try {
                 await localForageStorageForScope(scope).removeItem(persistedDisplayAccessKey(id, variant));