55 tests covering state machine transitions, heartbeat handling, push
deadline checks, group aggregation, history recording, probe aggregation,
log management, state management, and concurrency safety.
Checker tests cover HTTP (via httptest), port (via net.Listen),
isCodeAccepted ranges, and siteTimeout defaults. Ping and DNS
checkers skipped (need ICMP privileges and DNS server).
Coverage: 64.2% overall, 100% on handleStatusChange, triggerAlert,
checkPush, recordCheck, and AggregateStatus.
