test(matrix): share published realm across read-only host-mode tests

The host-mode beforeEach created a user, built a realm from ~7 card
sources, and published it (a full reindex + prerender fan-out) before
every test. With fullyParallel + 2 workers, those publish storms ran
concurrently and starved individual renders, which is what made the
page.goto navigations flaky.

Extract the setup into createAndPublishHostModeRealm and run it once
per worker via beforeAll for the read-only tests, which now share a
single published realm. The two routing-rule tests overwrite
realm.json and re-publish, so they keep an isolated per-test realm in
their own describe block. Also fold the duplicated publish + warm-up
polling into shared helpers (publishRealm, waitForPublishedMarker) and
warm up the page-title test's cold navigation the same way the print
test does.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: backspace