feat(web): serve workspace files from their own origin

A sandbox header bought isolation by taking the document's origin away, and
measuring that cost decided against it: the reported artifact throws
SecurityError on load, and because an uncaught exception aborts the rest of
its <script>, every listener declared after that line — theme toggle, mobile
menu, model tabs — never binds. Two of the four artifacts in the reporting
user's workspace were dead pages under it, and they still looked right.

A second listener on the API's host, answering /f and nothing else, is the
same boundary without the amputation: cross-origin to /api (refused by the
Origin fence and by CORS), same-origin with itself (localStorage, cookies and
fetch all work). Its port is published into the index page; the browser half
reads it to address previews, and its absence — the keyless fixture lane — is
what makes a file row fall back to the Host opener instead of a dead tab.

fileUrl moves from IWorkspaces to ConnectionHandle: the transport owns both
the listener that serves the bytes and the port that addresses it.

packages/client/runtime/tests/client-apply.spec.ts

@@ -26,6 +26,7 @@ async function mount(): Promise<Bench> {
   const bench: Bench = { ctx, api, sinks: undefined, stopped: 0 }
   const handle: ConnectionHandle = {
     api,
+    fileUrl: () => undefined,
     start: (sinks) => {
       bench.sinks = sinks
       return { stop: () => { bench.stopped += 1 } }