Vendor-real CI lane (or record/replay)

P1 backlog: a CI lane that runs the adapter suites against real vendors (or recorded cassettes) so vendor drift is caught before deploy instead of on prod. The fail-fast-on-0-tests guard (check-integration-ran.mjs) shipped 2026-06-30 as the first half. Candidate first target: STORAGE_DRIVER=s3 presign/serve lane (deferred from the upload-pipeline proof).