feat: add "use worker" directive for server Worker Threads and client Web Workers

[lazarv]

Summary

This PR introduces the "use worker" directive — a new framework-level primitive that lets developers offload computation to Node.js Worker Threads on the server and Web Workers in the browser using a single, unified API.

Features

"use worker" directive

• Add a "use worker" directive at the top of any module to mark all its exports as worker functions.
• At build time, the framework replaces exports with thin proxy stubs; the original code is moved into a virtual module that runs inside the worker thread.
• Works transparently on both server and client — the framework picks node:worker_threads or the browser Worker API automatically.

RSC Flight protocol serialization

• All arguments and return values are serialized via the React Server Components Flight protocol.
• Worker functions can return plain values, React elements, Suspense boundaries, Promises (for deferred rendering with use()), and ReadableStreams.
• ReadableStream values are transferred (zero-copy) between threads via postMessage transfer lists.

Server-side Worker Threads

• Workers are spawned lazily on first call and reused across requests.
• Automatic restart on crash.
• Worker context (AsyncLocalStorage) provides access to an abort signal for cancellation.
• Full integration with the framework's logging and dev server (Vite module runner support).

Client-side Web Workers

• Same "use worker" directive works in client components.
• The framework generates a Web Worker entry that imports the virtual module and wires up the message protocol.
• Transparent promise-based API — call worker functions as regular async functions.

after() API

• New after() server API (exported from @lazarv/react-server) that schedules callbacks to run after the response has been sent to the client.
• Useful for logging, analytics, cleanup, or fire-and-forget tasks.

Build & Dev integration

• New Vite plugin (react-server:use-worker) handles module transformation, virtual module resolution, and worker entry generation for both dev and production builds.
• Dev mode uses Vite's ModuleRunner inside workers for HMR-compatible execution.
• Production build emits worker chunks alongside the main server/client bundles.

Flight streaming improvements

• Enhanced toStream / fromStream helpers in @lazarv/react-server/rsc (and rsc/browser) to support streaming serialization of complex React trees, Promises, and ReadableStreams across thread boundaries.
• Comprehensive test suite (flight-streaming.test.mjs) covering primitives, nested objects, React elements, Suspense, error propagation, streams, and concurrent calls.

New files

Path	Description
packages/react-server/lib/plugins/use-worker.mjs	Vite plugin for "use worker" directive
packages/react-server/server/worker-proxy.mjs	Server-side worker thread manager
packages/react-server/server/worker-proxy-edge.mjs	Edge-compatible worker proxy stub
packages/react-server/server/worker-context.mjs	AsyncLocalStorage context for workers
packages/react-server/client/worker-proxy.mjs	Client-side Web Worker proxy
packages/react-server/server/after.mjs	after() API implementation
packages/react-server/lib/dev/worker.mjs	Dev-mode worker thread entry
packages/react-server/lib/start/worker.mjs	Production worker thread entry
docs/src/pages/en/(pages)/framework/worker.mdx	Documentation page
examples/use-worker/	Full example app demonstrating server & client workers
test/__test__/apps/use-worker.spec.mjs	Integration tests
packages/rsc/__tests__/flight-streaming.test.mjs	Flight protocol streaming tests

Modified files

• packages/rsc/client/shared.mjs / packages/rsc/server/shared.mjs — Enhanced Flight streaming helpers (toStream, fromStream) for cross-thread serialization.
• packages/react-server/server/index.mjs / index.d.ts — Export new after() and logger APIs.
• packages/react-server/server/symbols.mjs — New runtime symbols for worker context.
• packages/react-server/server/runtime.mjs — Runtime support for after() callbacks.
• packages/react-server/server/render-rsc.jsx / request.mjs — Wire up after() execution post-response.
• packages/react-server/lib/build/server.mjs / client.mjs — Include worker plugin in build pipeline.
• packages/react-server/lib/dev/create-server.mjs — Dev server worker thread setup.
• packages/react-server/lib/start/create-server.mjs — Production server worker setup.
• packages/react-server/lib/http/middleware.mjs — after() callback execution in HTTP middleware.
• packages/react-server/client/ClientProvider.jsx / error-overlay.mjs — Client-side worker integration.
• docs/ — Updated framework docs index and added worker documentation page.

[codecov-commenter]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
1361	1	1360	4

View the top 1 failed test(s) by shortest run time

__test__/apps/use-worker.spec.mjs > use worker > terminate worker and page recovers

Stack Traces | 33.8s run time

Error: page.goto: net::ERR_CONNECTION_REFUSED at http://localhost:4459/
Call log:
  - navigating to "http://localhost:4459/", waiting until "load"

 ❯ __test__/apps/use-worker.spec.mjs:79:16

To view more test analytics, go to the Test Analytics Dashboard
_{📋 Got 3 mins? Take this short survey to help us improve Test Analytics.}