Split the integration suite into four shards running two Playwright
workers each. Median wall time for the job drops from ~40 min to an
expected ~15 min; the build job is unchanged at ~4 min.
Shard reports are merged into a single HTML report, and the merged
run is summarised in the job step summary: totals, failed specs and
flaky specs ranked by retry count.
Chromium is installed into a shared volume so shards do not
re-download it. `workflow_dispatch` allows running the suite manually
against an arbitrary ref, with configurable shard layout and workers.
PRs targeting `staging` keep running serially while the current
release stabilizes. The exception is marked TEMPORARY and removed in
a follow-up.
Enable Playwright's JSON reporter alongside `list` and publish a
summary of flaky tests to the job step summary. The JSON report is
kept as an artifact for 30 days so flakiness rates can be aggregated
over time.
CI already runs with `retries: 2`, so unstable tests have been passing
silently on retry. This only surfaces what the suite already absorbs;
no test behaviour changes.
The reporter in `frontend/scripts/test-e2e` becomes overridable via
`PLAYWRIGHT_REPORTER` so the local developer default stays untouched.
