- COR-001: drain runHeartbeat goroutine via done channel before test returns,
eliminating race against concurrent SetHeartbeatInterval tests
- COR-002: replace time.After with time.NewTimer + defer t.Stop() + t.Reset()
to prevent timer goroutine leaks on context cancellation
- COR-005: guard HeartbeatInterval store write with > 0 check in OnClusterUpdated,
keeping store consistent with SetHeartbeatInterval's zero-ignore semantics
- add heartbeat.go with heartbeatIntervalSecs var, SetHeartbeatInterval, runHeartbeat (no build tag, CGO-free)
- add heartbeat_test.go with 5 tests covering interval validation and goroutine lifecycle
- add StartHeartbeat to clib.go (CGO impl calling xcecli_heartbeat)
- remove SetHeartbeatInterval stub from stub.go (now real code in heartbeat.go)
- remove TestSetHeartbeatInterval_StubNoOp from stub_funcs_test.go
- call SetHeartbeatInterval in OnClusterUpdated to track backend-pushed interval
- call backend.StartHeartbeat in cmd/main.go after backend registration
