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/heartbeat_test.go

@@ -0,0 +1,80 @@
+package backend
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestSetHeartbeatInterval_ValidValue(t *testing.T) {
+	atomic.StoreInt32(&heartbeatIntervalSecs, 30)
+	SetHeartbeatInterval(60)
+	if got := atomic.LoadInt32(&heartbeatIntervalSecs); got != 60 {
+		t.Errorf("heartbeatIntervalSecs = %d, want 60", got)
+	}
+}
+
+func TestSetHeartbeatInterval_ZeroIgnored(t *testing.T) {
+	atomic.StoreInt32(&heartbeatIntervalSecs, 30)
+	SetHeartbeatInterval(0)
+	if got := atomic.LoadInt32(&heartbeatIntervalSecs); got != 30 {
+		t.Errorf("heartbeatIntervalSecs = %d, want 30 (zero should be ignored)", got)
+	}
+}
+
+func TestSetHeartbeatInterval_NegativeIgnored(t *testing.T) {
+	atomic.StoreInt32(&heartbeatIntervalSecs, 30)
+	SetHeartbeatInterval(-1)
+	if got := atomic.LoadInt32(&heartbeatIntervalSecs); got != 30 {
+		t.Errorf("heartbeatIntervalSecs = %d, want 30 (negative should be ignored)", got)
+	}
+}
+
+func TestRunHeartbeat_CallsFunction(t *testing.T) {
+	atomic.StoreInt32(&heartbeatIntervalSecs, 1)
+
+	var callCount int64
+	fn := func(_ int) {
+		atomic.AddInt64(&callCount, 1)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go runHeartbeat(ctx, fn)
+
+	time.Sleep(1500 * time.Millisecond)
+	cancel()
+
+	if got := atomic.LoadInt64(&callCount); got < 1 {
+		t.Errorf("fn was called %d times, want at least 1", got)
+	}
+}
+
+func TestRunHeartbeat_CancelsOnContext(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // already cancelled
+
+	var callCount int64
+	fn := func(_ int) {
+		atomic.AddInt64(&callCount, 1)
+	}
+
+	done := make(chan struct{})
+	go func() {
+		runHeartbeat(ctx, fn)
+		close(done)
+	}()
+
+	select {
+	case <-done:
+		// returned quickly as expected
+	case <-time.After(100 * time.Millisecond):
+		t.Error("runHeartbeat did not return promptly after context cancellation")
+	}
+
+	if got := atomic.LoadInt64(&callCount); got != 0 {
+		t.Errorf("fn was called %d times, want 0 for already-cancelled context", got)
+	}
+}