ci: stop apt from gating the showcase Chromium install - #1034

Root cause

Showcase Tool Tests has been intermittently hanging. The logs from PR #1031 pin it down precisely — it is not the browser download, it is apt.

npx playwright install --with-deps chromium shells out to apt-get, which then stops responding against the Azure Ubuntu mirror:

Installing dependencies...
Switching to root user to install dependencies...
Ign:2 http://azure.archive.ubuntu.com/ubuntu noble InRelease
Ign:3 http://azure.archive.ubuntu.com/ubuntu noble-updates InRelease
...
##[error]The action 'Install Chromium' has timed out after 10 minutes.

Nothing was logged for the 9.5 minutes between the last InRelease line and the timeout.

Evidence it is the mirror and not the diff or the download:

• It stalled 21 minutes on chore(mix_generator): clear the DCM backlog in mix_generator and mix_annotations #1032, then again to the 10-minute bound on feat(mix): add content-sized GridTrack.auto() rows #1031
• Every other step in the job passed both times
• The same job passed in 1m59s and 2m13s on other runs in the same hour
• Neither PR's diff touches anything this job reads

The step bound added in #1033 is doing its job here: it converted a six-hour hang into a labelled 10-minute failure that names the culprit. This PR removes the cause.

Fix

Drop --with-deps. That flag exists to apt-get install Chromium's shared libraries. The ubuntu-24.04 runner image already ships them, and Playwright is pinned to 1.57.0 here — well past 1.45, where Ubuntu 24.04 support landed. This takes apt off the critical path entirely.

Cache ~/.cache/ms-playwright keyed on the lockfile. The browser only changes when the pinned Playwright version does, so most runs skip the download entirely.

Risk, and why it is the right trade

If a future runner image drops a library, Chromium fails to launch. That surfaces as a missing-library error naming the library, in seconds — and the fix is to apt-get install that one package, not to put all of apt back in front of every run. That is a strictly better failure mode than an unexplained hang. The reasoning is recorded in a comment on the step so it does not get "helpfully" reverted later.

Worth noting the job genuinely needs a browser: test/visual-report.test.mjs calls chromium.launch() to verify the generated report opens without console errors. So dropping the install altogether was not an option — only the apt half is unnecessary.

Verification

Deliberately proven in CI rather than argued: this PR's first run has a cache miss, so it exercises the real npx playwright install chromium path without --with-deps. If the job goes green, the removal is confirmed on the actual runner image.