[wieslawsoltes]

Problem

Active-listener retention covered same-realm and dedicated Worker MessagePorts, but explicit low-memory collection did not reach ServiceWorker isolates. Iframe transfers also treated a peer in another V8 context as a locally traceable peer, allowing the receiving wrapper to be reclaimed even though it had an active listener.

Change

• forward low-memory requests to every live ServiceWorker isolate on its serialized runtime thread
• compact child native state and request V8 collection before the next dispatch
• root an active MessagePort listener when its peer belongs to a different V8 context
• add forced-GC ServiceWorker coverage for both onmessage and addEventListener
• add a Code OSS-shaped iframe bootstrap gate that collects between listener installation and delivery
• bound the 100-cycle ServiceWorker gate by queue, binding, heap, RSS, and elapsed-time checks

Validation

• WEBSCENE_HYBRID_V8_RUNTIME_TEST_FILTER=messageport-iframe-gc webscene_hybrid_v8_runtime_tests
• WEBSCENE_NATIVE_ENGINE_TEST_FILTER=service-worker-messageport-gc webscene_native_engine_tests
• git diff --check

Closes #825