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.

script/test-e2e

@@ -35,10 +35,26 @@ main() {
     #   64MB /dev/shm or renderers crash.
     # --user: keep files the suite touches owned by the caller, not root.
     # HOME=/tmp: the mapped uid has no home directory in the image.
+    # PW_EXPERIMENTAL_SERVICE_WORKER_NETWORK_EVENTS=1: without it,
+    #   ctx.route() intercepts page requests only, and every fetch made by
+    #   the MV3 background service worker — including the phishing
+    #   blocklist fetch that src/background/index.js issues at worker
+    #   startup — goes to the real internet. The flag is experimental and
+    #   Playwright may drop or rename it. It cannot break silently: the
+    #   harness probes service-worker interception at launch and aborts
+    #   the whole suite if it is not in effect (see the interception
+    #   canary in tests/e2e/harness.js). If a future Playwright removes
+    #   the flag, that probe is what will fail, and the fix is either a
+    #   replacement mechanism or an honest downgrade of the isolation
+    #   claim in tests/e2e/network.js and README.md — not deleting the
+    #   probe. The image is pinned by digest, so this can only ever bite
+    #   on a deliberate bump.
     docker run --rm \
         --ipc=host \
         --user "$(id -u):$(id -g)" \
         -e HOME=/tmp \
+        -e PW_EXPERIMENTAL_SERVICE_WORKER_NETWORK_EVENTS=1 \
+        -e "E2E_TRACE_NETWORK=${E2E_TRACE_NETWORK:-0}" \
         -v "$ROOT:/work" \
         -w /work \
         "$IMAGE" \