HeartbeatService.Start() always returned early because running()
checked stopChan closure state, which is "open" (= true) for a
newly created service. This caused Start() to interpret a fresh
service as "already running" and skip launching the goroutine.
Introduce a `started` bool field to separate "has been started"
from "has not been stopped", fixing both the start failure and
a potential double-close panic on Stop().
