SLO error budget

An SLO of 99.9% availability over a 30-day window (2,592,000 s) grants an error budget of budgetSeconds = window × (1 − slo). Implement static double BudgetSeconds(double windowSeconds, double slo) and static (double remaining, bool within) Spend(double windowSeconds, double slo, double downtimeSeconds) returning remaining budget and whether downtime is still within it (within = downtime ≤ budget).