Stacked on #2040 (feat/dom-exception-serializable) — merge that first.

What this adds

Native MessagePort / MessageChannel / BroadcastChannel / MessageEvent (all lazy globals — zero boot cost), node:worker_threads, and Worker + worker global scope as real EventTargets.

The native core is Node's node_messaging design without libuv: an isolate-free PortData (mutex-guarded queue, sibling-group entanglement) under a per-isolate NativeMessagePort whose wake primitive is a coalesced EventLoop::PostInternal — producers never take a foreign isolate's Locker. Pairwise channels and named broadcast groups share one SiblingGroup mechanism (the pairwise-vs-broadcast close difference is a single guard, as in Node). Ports transfer through postMessage (including Worker.postMessage) and structuredClone as host-object tag 2: index in-stream, PortData out-of-band, nothing detached until the whole graph has serialized, received ports pre-constructed before ReadValue. A transferred port carries its queued backlog and drains after adoption on a later turn, per spec.

worker.onmessage / scope onmessage are now HTML event-handler IDL attributes (defineEventHandler, position-fixed ordering interleaving with addEventListener), and delivery dispatches real MessageEvents with event.ports populated. First message listener starts a port; receiveMessageOnPort does forced sync drains.

docs/worker-threads.md has the full real-vs-shim table and every documented deviation. Highlights: real MessageChannel/MessagePort/BroadcastChannel/receiveMessageOnPort/threadId/isMainThread/set-/getEnvironmentData/markAsUntransferable/markAsUncloneable; parentPort is a bridge; Worker is a thin emitter wrapper that rejects unsupported options loudly and forwards the rest of the option bag (so androidPriority reaches the runtime's own constructor); postMessageToThread/moveMessagePortToContext throw; locks absent.

Fixed in passing

• Worker error propagation (pre-existing bugs). CallWorkerScopeOnErrorHandle forwarded BOTH the scope handler's thrown error and the original. A throwing scope handler now forwards its own error once and nothing else, in all three worker error paths — the scope handler, the entry-rejection reporter in WorkerWrapper.cpp, and the unhandled-rejection tracker in NativeScriptException.cpp. A worker with no scope onerror still reaches the parent.
• Parent-side delivery is a real cancelable ErrorEvent on the Worker EventTarget, so worker.addEventListener('error') works, in registration order; handled = preventDefault() or a truthy onerror return. error is null (only primitives cross isolates); stackTrace is a documented NS extension. An error the Worker object leaves unhandled is dispatched as an ErrorEvent on the parent's global scope per HTML, and logged if nothing handles it there.
• EventLoop::Shutdown moves the dropped lanes out and destroys them after releasing the mutex. A dropped message carrying a transferred port sentinels the port's sibling, which posts to the sibling's loop; when that sibling belonged to the isolate shutting down, the post re-entered the held (non-recursive) mutex. The invariant is recorded in the class comment.
• ConcurrentQueue::Terminate empties the queue and destroys the messages outside both locks, and a push racing it is dropped under the queue mutex. Ports and buffers transferred to a worker terminated before its entry settled were pinned for the wrapper's lifetime and the sibling never received close.
• AbortSignal#onabort refactored onto the shared defineEventHandler (−42 lines).
• Post-write revalidation now covers buffers as well as ports: a getter that detached a listed ArrayBuffer while the graph was written used to hand the receiver zero bytes silently.
• Deserialize records which adopted ports the stream referenced; callers that surface no port list (structuredClone, receiveMessageOnPort) close the rest on arrival, and a read that fails after adoption closes every port it adopted.

Tests

Full device suite on a Pixel_3a_API_36 arm64 emulator: 1391 specs / 0 failures / 4 skipped (baseline before this change: 1216 / 0 / 4). npm run lint clean.

All five shared messaging suites ran (confirmed in the results XML, not pending): MessageChannel 45, MessageEvent 29, NodeWorkerThreads 36, BroadcastChannel 17, WorkerEvents 15. Plus 17 Android-only specs in tests/testMessaging.js (transfer-list edges, handler-attribute enabling, MessagePort.onclose, the empty-name BroadcastChannel group, the parentPort emitter surface, the two worker error paths, and the AbortSignal handler-attribute GC accounting) and 3 messaging canary specs in testRuntimeImplementedAPIs.js. The 4 skips are the pre-existing ones; the known __time flake did not fire.

Deviations from NativeScript/ios#454

• No g_states registry in Messaging.cpp. iOS needs a process-wide Isolate* -> MessagingState* map because its Caches is invalidated before CloseAllPorts runs. On Android Runtime::DestroyRuntime releases RuntimeState in its very last statement, long after CloseAllPorts, so RuntimeState::For<MessagingState> answers there and the registry (plus the isolate field and the registry-erasing half of ~MessagingState) is dropped.
• IsolateWrapper -> a raw v8::Isolate* guarded by Runtime::TryGetRuntime, which is Android's "is this runtime still alive" primitive.
• Uncaught listener errors from a port drain and from parent-side worker delivery go through Android's ContainUncaughtCallbackException + EventLoop::IsPumping()/DeferJavaThrow/ReThrowToJava tail, mirroring Timers.cpp, rather than iOS's ReportToJsHandlersAndLog. Android's internal lane performs a microtask checkpoint after each entry, so an exception may not be left pending across the return.
• Worker::InitEvents/EmitError/OnMessageCallback live in a new WorkerEvents.{h,cpp} — Android has no Worker.{h,mm} counterpart; WorkerWrapper keeps thread lifecycle only.
• eslint restrictedGlobals gains Promise and WeakSet but not WeakMap (iOS added all three): Android's primordials exports no WeakMap and no builtin uses one, so the rule would be unsatisfiable.
• The "forwards the option bag" spec uses androidPriority: "turbo" with .toThrow() where iOS uses resourceLimits with toThrowError(TypeError). Android's native Worker raises a NativeScriptException, not a TypeError; the assertion's intent (the bag reaches the native constructor) is unchanged.
• structuredClone's native half needed no change: Deserialize with a null port list already closes ports that arrive with no way out.

Remaining follow-ups (out of scope)

• messageerror relay is wired but has no end-to-end test (no deterministic way to force a deserialization failure from JS).
• The Worker lifetime work (nsworkerended, terminate() settling) mirroring fix(runtime): strong Worker wrapper lifetime while the thread runs ios#456 is the next PR in this stack.

Mirrors NativeScript/ios#454.

Summary by CodeRabbit

• New Features

  • Added support for messaging APIs, including message channels and ports, broadcast channels, and worker messaging events.
  • Added partial node:worker_threads compatibility, with support for common worker and parent-port messaging features.
  • structuredClone can now transfer MessagePorts alongside ArrayBuffers.
  • Added documentation on worker messaging, transfer behavior, and supported worker-thread APIs.

• Bug Fixes

  • Improved worker error reporting so errors thrown by handlers are reported accurately.
  • Improved message and event-loop shutdown handling.