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.