docs: document running the end-to-end suite without root

[mrbobbytables]

Finding (#1277)

CONTRIBUTING.md documented one way to get Playwright's OS packages, npx playwright install --with-deps chromium, which runs apt-get install and so needs root. Without root the recipe cannot be followed, and the failure it leaves (browserType.launch: Target page, context or browser has been closed) names neither the missing library nor the install step. Three earlier findings (#1232, #1253, #1256) each had to note the browser suite could not be run locally for exactly this reason.

What changes

• CONTRIBUTING.md ("End-to-end tests"): adds the non-root recipe beside the existing one. Install the browser bundle alone, apt-get download the packages into a private sysroot (private APT state dir so no root is touched), dpkg-deb -x them, and point LD_LIBRARY_PATH at the result. The package list is the corrected one from the issue thread: the 28 chrome-headless-shell needs plus the 17 more the full chrome binary loads, so headed and channel: 'chromium' runs work too. The ldd check loops over both binaries. Notes cover the harmless rm: cannot remove .../partial noise, the per-shell export, not setting XDG_CACHE_HOME, and the arm64 loader path. The package block is fenced with cspell:disable/enable rather than adding 40 package names to the dictionary.
• tests/contributing-e2e-setup.test.mjs: the contract treated any npx playwright install without --with-deps as incomplete unless the doc also named install-deps (which needs root just the same). It now also accepts a doc that unpacks the packages into a sysroot and hands the loader that directory (documentsSystemPackages), with a fixture test pinning that neither half alone passes. Without this change the test rejects the new section (CONTRIBUTING.md: npx playwright install chromium), so it is a genuine contract update rather than a loophole.

Verification

Ran the recipe as an unprivileged user on Ubuntu 26.04 (the issue verified Debian trixie):

• all 45 packages download; both chrome and chrome-headless-shell report 0 missing under ldd (from 5 and 4 before);
• npm run build:production && npm run test:e2e with the sysroot on LD_LIBRARY_PATH: 312 passed, 30 skipped (1.2m).
• tests/contributing-e2e-setup.test.mjs + tests/dev-environment.test.mjs: 20 pass. check:format, check:spelling, markdownlint on CONTRIBUTING.md: clean.

Closes #1277