def canary_decision(stable, canary, min_improvement, max_latency_increase):
    return 'promote' | 'hold' | 'rollback'

1. rollback unsafe or slower candidates
2. promote only when quality clears the practical lift
3. hold inconclusive candidates

• Use only the Python standard library.
• Return deterministic values so every run can be compared with the contract.
• Do not mutate caller-owned inputs.