CI: browser tests run one engine at a time since Vitest 5.0.4 #86

Context

Vitest 5.0.4 (#83) runs each browser instance as a worker ("Run browser instances as workers", vitest-dev/vitest#11525). The browser project sets fileParallelism: false, because its files share an origin and would leak localStorage and IndexedDB into each other. With 5.0.4 that also means one browser at a time: the instances no longer run side by side.

Measured on main merged with #83, BROWSERS=chromium,webkit, warm:

• whole suite: 49 s on 5.0.3, 79 s on 5.0.4. Same tests: 1,292 passed, 6 skipped.
• --maxWorkers=2: no change (76 s).
• one project per browser, each with fileParallelism: false: no change (76 s).

In CI, the Browsers job, which runs all three engines, went from 2m02s (#84) to 3m19s (#83).

Deliverable

• Run the engines in parallel again, while each engine still runs one file at a time. For example, make the browsers job in .github/workflows/ci.yml a matrix over chromium, firefox and webkit, each with its own BROWSERS and installing only its own browser.
• Or, if Vitest has a setting for this, use it in vitest.config.ts instead, and say which one.

Acceptance criteria

• The Browsers check runs every engine, with the same test count per engine as before.
• Its wall-clock time is back near the 5.0.3 figure (about 2 minutes), measured over at least 2 runs.
• A failure still names the engine it failed in.