Browser E2E harness for the mock host + `createMockClient`

Context

#257 (delivered by PR #258) added the in-core mock host: MockPlatform (Rust) and createMockHost (JS), proven through the real dispatcher and a headless wasm-bridge.test.ts that drives the real WASM core across the JS↔SCALE↔WASM boundary. By design that test uses no real Web Worker, no iframe, and no product transport — so the highest-fidelity path is still unproven: a real product, in a real browser, talking to the real core over the production transport. Adopting the mock in a product test is also more setup than it should be (createMockHost + provider + createClient by hand).

Goal

Prove the mock host end to end in a real browser — the real truapi-server WASM core in a Web Worker, a product in an iframe connecting via the SDK's real getClientSync() sandbox path over a real MessageChannel, driven headless by Playwright — and make "switch a product to mock mode" a one-liner so product tests adopt it trivially.

Proposed shape

@parity/truapi-mock-e2e — a private (unpublished) workspace package holding the browser harness. Two topologies:

• single-page (src/main.tsx): host + product in one page, wired with a single createMockClient() call;
• iframe (src/host.tsx + src/product.tsx): mirrors production embedding — the host boots the core + createIframeHost; the product runs in an iframe with no mock code, connecting through getClientSync().

Dependencies resolve through the npm workspace (no machine-specific paths), so it runs anywhere the repo does.

createMockClient — a @parity/truapi-host-wasm/testing subpath export that collapses createMockHost + createWebWorkerProvider + createClient into one call, returning the product client plus the mock for assertions:

import HostWorker from "@parity/truapi-host-wasm/worker-runtime?worker";
import { createMockClient } from "@parity/truapi-host-wasm/testing";

const { client, mock } = await createMockClient(new HostWorker(), { devicePermissions: "allow-all" });
await client.system.handshake();
// ... drive the product client; assert via mock.navigations(), etc.

Scope

• @parity/truapi-mock-e2e package: Vite + React harness (single-page + iframe topologies), portable workspace file: deps.
• createMockClient (+ MockClient type) exported from @parity/truapi-host-wasm/testing.
• Playwright suite: (1) product-in-iframe round-trip through the real core; (2) a single-page test that executes createMockClient at runtime. Assert on both product results and the mock's recorded oracles (e.g. navigations()).
• A mock-e2e CI job: build the WASM + workspace packages, install Chromium, run Playwright. Runs for signal; not a required ci-status gate until it has a green baseline in CI.
• README docs for the new package and the /testing export.

Acceptance criteria

• The suite passes headlessly with a real Web Worker and real iframe (not a stubbed transport).
• The product reaches the core only through the SDK's real getClientSync() sandbox path; the product contains no mock code.
• createMockClient has executing runtime coverage, not just typecheck.
• No dependency on a device or a live chain; the run is deterministic and repeatable.
• The harness is dev-only / private and never enters a production build.

Gating

Stacked on #257 / PR #258 (createMockHost, the providers, and the /testing export live there)