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/src/client/contract/workspaces.ts

@@ -56,17 +56,6 @@ export interface IWorkspaces {
    * @param path - absolute or host-resolvable path.
    */
   openPath(path: string): Promise<void>
-  /**
-   * URL serving one file out of a session's workspace, for a UI that opens a
-   * produced file in the browser instead of on the Host machine.
-   * @param sessionId - the session whose cwd anchors the path.
-   * @param cwd - that session's working directory, or `undefined` when unknown.
-   * @param path - the path a tool reported (absolute, or relative to `cwd`).
-   * @returns the origin-relative URL, or `undefined` when the path lies
-   * outside the workspace — which this transport never serves, leaving
-   * {@link IWorkspaces.openPath} as the only way to reach it.
-   */
-  fileUrl(sessionId: SessionId, cwd: string | undefined, path: string): string | undefined
   /**
    * Rename a Workspace.
    * @param workspaceId - target workspace.

packages/client/runtime/src/client/workspaces/service.ts

@@ -5,7 +5,6 @@ import type {
   DirectoryListing, IApiClient, RpcError,
   SessionId, WorkspaceId, WorkspaceView,
 } from '@deepseek-ai/dsh-client-connection/client'
-import { workspaceFileSegments, workspaceFileUrl } from '@deepseek-ai/dsh-host-apiproxy/api'
 import type { SnapshotStore } from '../contract/store.ts'
 import { createSnapshotStore } from '../contract/store.ts'
 import type { SessionsPort, SessionsPortList } from '../contract/sessions-port.ts'
@@ -240,18 +239,6 @@ export class WorkspacesService implements IWorkspaces {
     }
   }
 
-  /**
-   * URL serving one file out of a session's workspace.
-   * @param sessionId - the session whose cwd anchors the path.
-   * @param cwd - that session's working directory, or `undefined` when unknown.
-   * @param path - the path a tool reported (absolute, or relative to `cwd`).
-   * @returns the origin-relative URL, or `undefined` for a path outside the workspace.
-   */
-  fileUrl(sessionId: SessionId, cwd: string | undefined, path: string): string | undefined {
-    const segments = workspaceFileSegments(cwd, path)
-    if (segments === undefined) return undefined
-    return workspaceFileUrl(sessionId, segments)
-  }
 
   /**
    * Rename a Workspace.

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 } }

packages/client/runtime/tests/wire-events.spec.ts

@@ -20,6 +20,7 @@ async function mount(): Promise<Bench> {
   const bench: Bench = { ctx, sinks: undefined }
   const handle: ConnectionHandle = {
     api,
+    fileUrl: () => undefined,
     start: (sinks) => {
       bench.sinks = sinks
       return { stop: () => {} }

packages/client/runtime/tests/workspaces-service.spec.ts

@@ -276,21 +276,6 @@ describe('WorkspacesService', () => {
     await expect(workspaces.openPath('/missing')).rejects.toThrow(/path open failed/)
   })
 
-  it('addresses a workspace file by URL, and only inside the workspace', async () => {
-    const ctx = new Context()
-    const api = new FakeApiClient()
-    const sessions = new SessionsService(ctx, api)
-    const workspaces = new WorkspacesService(ctx, api, sessions)
-    const session = 's-1' as SessionId
-    // The URL is derived, not fetched: no wire call answers a link.
-    expect(workspaces.fileUrl(session, '/w/alpha', '/w/alpha/out/a b.html')).toBe('/f/s-1/out/a%20b.html')
-    expect(workspaces.fileUrl(session, '/w/alpha', 'out/index.html')).toBe('/f/s-1/out/index.html')
-    // Outside the workspace there is nothing this transport may serve, which
-    // is the signal a caller falls back to openPath on.
-    expect(workspaces.fileUrl(session, '/w/alpha', '/etc/hosts')).toBeUndefined()
-    expect(api.calls).toHaveLength(0)
-  })
-
   it('deletes a Workspace or preserves it when the Host rejects deletion', async () => {
     const ctx = new Context()
     const api = new FakeApiClient()