Expose Worker in hydrated iframe realms

[wieslawsoltes]

Problem and change

The core Worker transport works in the top-level realm, but Code OSS creates its browser extension host inside a hydrated same-origin iframe. That realm needs its own Worker constructor, realm-local EventTarget chain, and URL base. Module workers also need a microtask checkpoint so a blob bootstrap using top-level await import(...) can finish before the transferred-port handoff.

This layer installs Worker in each hydrated iframe realm and adds a Code OSS-shaped contract for:

1. same-origin iframe creation;
2. blob-backed module Worker construction inside that iframe;
3. top-level dynamic import;
4. Worker startup; and
5. transferred MessagePort request/response.

It also makes the detached-iframe Inspector regression count callbacks only after the synchronous removal boundary, avoiding a race with legal delivery before detachment.

Focused validation on the cumulative stack:

• iframe module-worker extension-host contract: 4/4 assertions;
• WEBSCENE_NATIVE_ENGINE_TEST_FILTER=iframe-worker-inspector-detach with Inspector enabled: passed;
• cumulative worker/MessagePort hybrid filter: passed.

Stack: #172 -> #173 -> #174 -> #175. This is layer 2 of 4, based on #172. Reconstructs the focused behavior from #95 on current main.

Advances #81.