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/test-runtime/src/connection.ts

@@ -0,0 +1,48 @@
+/** Test-owned connection face: the transport members features read off `ctx.connection`. */
+import { workspaceFileSegments, workspaceFileUrl } from '@deepseek-ai/dsh-host-apiproxy/api'
+import type { ConnectionHandle, IApiClient, SessionId } from '@deepseek-ai/dsh-client-connection/client'
+
+/**
+ * Connection test double. Implements the same `ConnectionHandle` face features
+ * receive as `ctx.connection`, so a production face change breaks this double
+ * at compile time. The wire client is not modelled — a feature that needs one
+ * composes its own connection over a fake api client; this double exists for
+ * the transport facts features read synchronously, above all the
+ * workspace-file URL.
+ */
+export class TestConnection implements ConnectionHandle {
+  /**
+   * The workspace-file port the host would have published into the page.
+   * Unset — the default, and the keyless fixture lane's real state — makes
+   * {@link TestConnection.fileUrl} answer `undefined`, which is the signal a
+   * caller falls back to the Host opener on.
+   */
+  filesPort: number | undefined
+
+  /** The wire client; unused by this double's consumers and absent by construction. */
+  readonly api: IApiClient = undefined as unknown as IApiClient
+
+  /**
+   * Stream-loop starter (inert).
+   * @returns a stop handle that does nothing.
+   */
+  start(): { stop(): void } {
+    return { stop: () => {} }
+  }
+
+  /**
+   * Workspace-file URL, deriving exactly as production does so a feature test
+   * sees the real inside/outside-workspace split.
+   * @param sessionId - the Session whose cwd anchors the path.
+   * @param cwd - that Session's working directory.
+   * @param path - the path a tool reported.
+   * @returns the absolute URL on the workspace-file origin, or undefined when
+   * the path leaves the workspace or no port is published.
+   */
+  fileUrl(sessionId: SessionId, cwd: string | undefined, path: string): string | undefined {
+    if (this.filesPort === undefined) return undefined
+    const segments = workspaceFileSegments(cwd, path)
+    if (segments === undefined) return undefined
+    return `http://localhost:${String(this.filesPort)}${workspaceFileUrl(sessionId, segments)}`
+  }
+}

packages/client/test-runtime/src/index.ts

@@ -29,11 +29,13 @@ import type {
 } from '@deepseek-ai/dsh-client-ui-slots'
 import { registerDomSnapshotSerializer } from './snapshot.ts'
 import { TestSessions } from './sessions.ts'
+import { TestConnection } from './connection.ts'
 import { TestWorkspaces } from './workspaces.ts'
 import type { Stabilizer } from './fixtures.ts'
 
 export { domSnapshotSerializer, registerDomSnapshotSerializer } from './snapshot.ts'
 export { FixtureSession, TestSessions } from './sessions.ts'
+export { TestConnection } from './connection.ts'
 export { TestWorkspaces } from './workspaces.ts'
 export { conversationSnapshot, workspaceListState } from './fixtures.ts'
 export type { SessionBehaviorOverrides, SessionFixture, Stabilizer } from './fixtures.ts'
@@ -175,6 +177,8 @@ export class SlotTestRuntime {
   readonly sessions: TestSessions
   /** Workspaces double (list observable, recorded intent actions). */
   readonly workspaces: TestWorkspaces
+  /** The transport double features read as `ctx.connection`. */
+  readonly connection: TestConnection
 
   private readonly stabilizer: Stabilizer = async (fn) => {
     await act(async () => { await fn() })
@@ -195,8 +199,10 @@ export class SlotTestRuntime {
     this.root = new TestRoot(slots, this.stabilizer)
     this.sessions = new TestSessions(this.stabilizer, ctx)
     this.workspaces = new TestWorkspaces(this.stabilizer)
+    this.connection = new TestConnection()
     ctx.provide('sessions', this.sessions)
     ctx.provide('workspaces', this.workspaces)
+    ctx.provide('connection', this.connection)
     // Capturing install: the production renderer does the rendering; the
     // wrapper only takes the host face for storeOf (no machinery copied).
     const renderer = createSlotRenderer()

packages/client/test-runtime/src/workspaces.ts

@@ -1,6 +1,5 @@
 /** Test-owned workspaces face: the renderer standard-kit observable plus recorded actions. */
 import { createSnapshotStore } from '@deepseek-ai/dsh-client-runtime/client'
-import { workspaceFileSegments, workspaceFileUrl } from '@deepseek-ai/dsh-host-apiproxy/api'
 import type {
   DirectoryListing, IWorkspaces, SessionId, SnapshotStore, WorkspaceId, WorkspaceListState, WorkspaceView,
 } from '@deepseek-ai/dsh-client-runtime/client'
@@ -99,21 +98,6 @@ export class TestWorkspaces implements IWorkspaces {
     await (this.stubs.get('openPath')?.(path) as Promise<void> | undefined)
   }
 
-  /**
-   * Workspace-file URL (recorded). Runs the production path derivation so a
-   * feature test sees the real in/outside-workspace split; stub to force either.
-   * @param sessionId - the session whose cwd anchors the path.
-   * @param cwd - that session's working directory.
-   * @param path - the path a tool reported.
-   * @returns the origin-relative URL, or undefined outside the workspace.
-   */
-  fileUrl(sessionId: SessionId, cwd: string | undefined, path: string): string | undefined {
-    this.calls.push({ method: 'fileUrl', args: [sessionId, cwd, path] })
-    const stub = this.stubs.get('fileUrl')
-    if (stub !== undefined) return stub(sessionId, cwd, path) as string | undefined
-    const segments = workspaceFileSegments(cwd, path)
-    return segments === undefined ? undefined : workspaceFileUrl(sessionId, segments)
-  }
 
   /**
    * Directory picker (recorded). The default cancels (null); stub to select.