Apply the winning variant’s config to the gate

Use when: caller has decided on the winner of a split experiment and wants Layer to write the winning variant’s config back to the gate (graduating the test into production)

Take a completed or stopped experiment’s winning variant config and write it back to the gate. Snapshots the previous gate config into gate history so the change is reversible via rollback. Updates the experiment row with status: 'completed' and winner_variant: <A|B>. Records the outcome via recordExperimentOutcome for the Architect’s reward-signal aggregation.