[Ji-One1]

The scenario needs one owner for execution order, deadlines, and cleanup. Browser or buyer failures must not become false storefront failures.

What this changes

• Add a driver-based runner: open the storefront, record configured context, prepare the scenario, open its same-origin start page, create the buyer, and run checkpoints.
• Pass the scenario's selected Standard Events to the driver. Skipped preparation creates no buyer and makes no model call.
• Fail a pinned product route that returns HTTP 404 or 410 without a redirect. Other load problems remain inconclusive.
• Keep checkout evidence diagnostic: it does not abort the buyer. Step limits, run deadlines, and external cancellation still apply.
• Detach the buyer before the final checkpoint assertion and after an abort, error, or step limit. Keep a buyer whose earlier run ended with reason completed for the next checkpoint.
• Always offer the configured driver cart reader to assertions, regardless of checkout evidence. A missing driver reader remains unavailable; an unreadable page can still make a cart read fail.
• Bound assertions and cleanup, handle late-created sessions, append evidence in order, and write the final result. Cleanup and evidence-write problems are warnings; an unconfirmed required detach prevents a conclusive verdict.

Developer impact

Internal tooling only; no published Hydrogen changes or changeset. The runner accepts driver factories but does not yet supply a concrete browser/buyer driver or CLI.

Risk

Country and language remain configured-only values. Cleanup deadlines can leave work unfinished. Artifact write failures can prevent a complete saved report; result writing itself can reject. Private evidence is not redacted.

Stack: 06/15. Previous: #4313. Next: #4315.