Run service worker globals on worker threads #207

Build the ServiceWorkerGlobalScope execution path on top of the Phase 19 worker infrastructure.

Scope:

• Add ServiceWorker and ServiceWorkerGlobalScope JS objects with self.registration and basic state exposure.
• Start install/waiting/active service worker threads from persisted registrations.
• Dispatch install, activate, message, error, and lifecycle events to the dedicated worker thread.
• Implement skipWaiting() and clients.claim() plumbing sufficient for later fetch/client issues.
• Track outstanding extendable work so workers can shut down deterministically when idle.

Acceptance criteria:

• Tests cover service worker thread startup, install/activate event ordering, skipWaiting promotion, and error propagation.
• A registration from the previous issue can execute a simple worker script and transition to active.
• Implementation reuses worker/event/structured-clone infrastructure instead of duplicating it.