Add the Phase 21 regression suite once the core service worker implementation is in place.
Scope:
- Add focused WPT coverage entries for registration lifecycle, FetchEvent interception, Cache API behavior, client messaging, and update flow.
- Add e2e scenarios for online registration, offline reload from cache, controlled subresource fetch, and document/service-worker postMessage round trip.
- Extend the e2e harness only as needed for restart/offline/cache inspection controls.
- Document the new service worker test commands and artifact expectations in CLAUDE.md if the workflow changes.
Acceptance criteria:
- cargo test --workspace includes deterministic service-worker unit/integration coverage.
- cargo run -p we-e2e -- --scenario passes offline without external network.
- The smoke suite remains passing and the new scenarios would fail against a build without service worker support.