Shared-provider signals from official status pages

Rule cs-v1: when two or more companies that share a cloud region, hosting provider, CDN or DNS provider declare incidents on their own status pages within 60 minutes and the chance of that by coincidence is below 1 in 1,000 (Poisson-binomial on each company's 84-day incident rate, hour-of-week adjusted), the shared provider is flagged. Includes active signals, the last 30 days and the 84-day backtest with recall against provider records.