Run test:popups in under a minute

The suite took 13 minutes: all 36 tests in three engines, one at a time, and
one test walking all 162 stories with a fixed 400ms after each close. It now
runs like test:a11y, in parallel on a context shared per worker, with the story
sweep split into eight parts (takeStoryPart, now shared with the Axe sweep). It
also waits for events instead of time: an opening popup is checked every 20ms
rather than in 100/250/500ms steps, and a closing one is done when it leaves
the DOM, which the 250ms hide timeout does. Sign-in cards and the in-app
browser give no such signal and keep the 400ms.

Its server is now the e2e one (vite.e2e.config.ts). The popups config had kept
hot reload and the file watcher, so an edit during a run could reload a page
under a test. Nothing it serves changes during a run, so modules are cached for
the run and a worker's later pages boot from the browser cache.

A run is Chromium only, ~50 s. Firefox and WebKit boot the whole client for
each test (Firefox ~12 s a test), so they moved to test:popups:full
(POPUPS_ALL_ENGINES=1, implied on CI), ~3 min.

AGENTS.md: heavy suites run only at the final review and only when they can
observe the change; the new timings.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>