feat: P1-3 add heartbeat keep-alive goroutine

- 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

src/frontend/apiserver/backend/handler.go

@@ -64,6 +64,7 @@ func (h *DefaultHandler) OnClusterUpdated(incoming *models.ClusterInfo) {
 	existing.Vcpus = incoming.Vcpus
 	existing.InactiveHeartbeatCount = incoming.InactiveHeartbeatCount
 	store.Clusters.Put(existing.ClusterName, existing)
+	SetHeartbeatInterval(incoming.HeartbeatInterval)
 }
 
 func (h *DefaultHandler) OnResourceUpdated(res *models.ResourceDefinitionInfo) {