test: intercept the MV3 service worker's network in the e2e harness

ctx.route() does not see requests made by the background service worker
unless Playwright is run with PW_EXPERIMENTAL_SERVICE_WORKER_NETWORK_EVENTS=1,
so the phishing blocklist fetch that src/background/index.js issues at
worker startup was reaching raw.githubusercontent.com on the real
internet on every run. phishingDomains.js swallows fetch failures, so
nothing surfaced it, and the raw.githubusercontent.com stub in
tests/e2e/network.js was unreachable code that made the gap look covered.

script/test-e2e now sets the flag, with a comment recording what to do if
a future Playwright drops it. The flag being experimental is not taken on
trust: launch() waits for the worker's own startup request to arrive in
the route handler and refuses to run the suite if it never does, so
escaping traffic fails the run instead of passing unnoticed. Chrome is
additionally started with --host-resolver-rules=MAP * ~NOTFOUND, so
anything that does slip past interception cannot reach a real host.

Also: errors and unstubbed requests recorded during launch are attributed
to the first test rather than discarded, a suite that registers no tests
now fails instead of exiting 0, a failure after the browser is up tears
the context down instead of hanging the process, E2E_TRACE_NETWORK=1
prints every routed request tagged [sw] or [page], and the dead exports in
harness.js and network.js are gone.

README.md

@@ -101,6 +101,16 @@ intercepted at the browser level and served from fixtures in
 unrecognised outbound requests are reported as failures rather than silently
 allowed.
 
+That interception covers the MV3 background service worker as well as the popup
+page, which it does not by default — `script/test-e2e` sets
+`PW_EXPERIMENTAL_SERVICE_WORKER_NETWORK_EVENTS=1` for it. Because that flag is
+experimental, the harness does not take it on trust: at launch it fetches a
+`.invalid` URL from inside the service worker that only the route handler can
+answer, and aborts the entire suite if the answer does not come back. Escaping
+traffic fails the run instead of disappearing. To see what is actually being
+intercepted, run with `E2E_TRACE_NETWORK=1` and every routed request is printed,
+tagged `[sw]` or `[page]`.
+
 **Any uncaught page error or `console.error` fails the run.** That is the point:
 a `ReferenceError` from a used-but-not-imported identifier is invisible to
 `make check` (`script/lint` is only `prettier --check`) but fatal in a browser,