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/clib.go

@@ -64,6 +64,7 @@ import "C"
 import (
 	"apiserver/models"
 	logger "apiserver/utils"
+	"context"
 	"fmt"
 	"sync"
 	"unsafe"
@@ -153,6 +154,14 @@ func Cleanup() {
 	C.idgen_ctx_fini(&gIDCtx)
 }
 
+// StartHeartbeat launches a goroutine that sends periodic xcecli_heartbeat
+// keep-alive messages to the controller. Stops when ctx is cancelled.
+func StartHeartbeat(ctx context.Context) {
+	go runHeartbeat(ctx, func(secs int) {
+		C.xcecli_heartbeat(gCtx.ctx, C.int(secs))
+	})
+}
+
 // go_push_handler is called by the C library on every incoming request.
 // It must be exported so the C trampoline can call it from a C thread.
 //