Constant SQL_IDEMPOTENCY_MAX_FUTURE_SKEW_MS 

pub const SQL_IDEMPOTENCY_MAX_FUTURE_SKEW_MS: i64 = _; // 300_000i64

Public idempotency keys carry their mint time so an evicted outcome can fail closed without retaining one tombstone per operation. A modest future-skew allowance accommodates ordinary client clock drift while bounding how far one forged key can ratchet the durable floor.