Implement Worker, transferable MessagePort, and iframe transport

[wieslawsoltes]

Problem and change

Code OSS and Monaco need isolated workers plus a transferable MessagePort bootstrap path. Current main does not expose that transport, so browser extension hosts and language workers cannot use their normal execution model.

This base layer adds:

• classic and module dedicated workers using isolated V8 runtimes;
• Worker, MessageChannel, and MessagePort event behavior;
• structured clone for cycles, built-ins, ArrayBuffer, and transferred ports;
• same-origin iframe postMessage with explicit unsupported-navigation rejection;
• worker failure, termination, navigation, and shutdown handling;
• bounded worker and port queues, a 16 MiB message limit, worker and port binding limits, and transfer-before-detach validation;
• weak MessagePort wrapper ownership and reusable native binding slots.

Focused regressions cover ordering, transfer ownership, clone failures, queue saturation, prompt termination, navigation cancellation, receiver-side messageerror, and same-origin iframe port handoff. The performance gate completes 10,000 worker/port round trips within 10 seconds. The memory gate releases three batches of 2,048 channels and bounds the final V8 heap to 16 MiB above the warmed baseline.

Local validation on the cumulative stack:

• WEBSCENE_HYBRID_V8_RUNTIME_TEST_FILTER=worker-messageport: passed;
• directly related Worker/MessagePort WPT: 8/8 documents, 35/35 assertions;
• git diff --check: passed.

Stack: #172 -> #173 -> #174 -> #175. This is layer 1 of 4; #175 is the cumulative validation head.

Advances #81.