Load layouts and optional features from staged asset caches

frontend/direct-recv-worker.js

@@ -1,11 +1,11 @@
 'use strict';
-importScripts('/sha256.js');
 let dir, handle, access, hash, pos = 0, claim, name;
 let chain = Promise.resolve();
 self.onmessage = e => { chain = chain.then(() => run(e.data)); };
 async function close() { if (access) { access.flush(); access.close(); access = null; } }
 async function run(m) {
   try {
+    if (!self.Sha256) { self.__ASSET_URLS__=m.assetUrls || {}; importScripts(self.__ASSET_URLS__.sha256 || '/sha256.js'); }
     if (m.op === 'open') {
       claim = m.file;
       if (!/^[A-Za-z0-9_-]{1,128}$/.test(claim.id) || !/^[a-f0-9]{64}$/.test(claim.cid)) throw Error('Invalid file identity');