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.
