feat: P0-2d add no-CGO stubs for heartbeat, select, cleanup

Add four function stubs to backend/stub.go for the !cgo build path:
- StartHeartbeat(ctx context.Context) — no-op (heartbeat not available)
- SetHeartbeatInterval(secs int) — no-op (interval update not available)
- EvalSelectForHost(hostname, selectExpr string) error — returns errNoCGO
- Cleanup() — no-op (no C resources to release without CGO)

TDD: four tests in stub_funcs_test.go verify stubs behave correctly
(no-op functions don't panic, error stubs return errNoCGO). All backend
tests passing.

src/frontend/apiserver/backend/stub.go

@@ -4,6 +4,7 @@ package backend
 
 import (
 	"apiserver/models"
+	"context"
 	"errors"
 )
 
@@ -31,3 +32,15 @@ func ControlQueue(queuename string, action int) error { return errNoCGO }
 
 // ControlHost stub — real implementation in clib.go (requires CGO).
 func ControlHost(hostname string, action int) error { return errNoCGO }
+
+// StartHeartbeat stub — real implementation in clib.go (requires CGO).
+func StartHeartbeat(_ context.Context) {}
+
+// SetHeartbeatInterval stub — real implementation in clib.go (requires CGO).
+func SetHeartbeatInterval(_ int) {}
+
+// EvalSelectForHost stub — real implementation in clib.go (requires CGO).
+func EvalSelectForHost(_, _ string) error { return errNoCGO }
+
+// Cleanup stub — real implementation in clib.go (requires CGO).
+func Cleanup() {}

src/frontend/apiserver/backend/stub_funcs_test.go

@@ -0,0 +1,41 @@
+//go:build !cgo
+
+package backend
+
+import (
+	"context"
+	"testing"
+)
+
+// TestStartHeartbeat_StubNoOp verifies the no-CGO stub for StartHeartbeat
+// is a no-op that does not panic or return an error.
+func TestStartHeartbeat_StubNoOp(t *testing.T) {
+	// Should not panic and completes immediately
+	StartHeartbeat(context.Background())
+}
+
+// TestSetHeartbeatInterval_StubNoOp verifies the no-CGO stub for
+// SetHeartbeatInterval is a no-op that does not panic.
+func TestSetHeartbeatInterval_StubNoOp(t *testing.T) {
+	// Should not panic and completes immediately
+	SetHeartbeatInterval(30)
+	SetHeartbeatInterval(0)
+	SetHeartbeatInterval(-1)
+}
+
+// TestEvalSelectForHost_StubErrNoCGO verifies the no-CGO stub for
+// EvalSelectForHost returns errNoCGO, blocking callers from using
+// select expressions without the real CGO backend.
+func TestEvalSelectForHost_StubErrNoCGO(t *testing.T) {
+	err := EvalSelectForHost("host01", "mem>4")
+	if err != errNoCGO {
+		t.Errorf("EvalSelectForHost stub should return errNoCGO, got %v", err)
+	}
+}
+
+// TestCleanup_StubNoOp verifies the no-CGO stub for Cleanup
+// is a no-op that does not panic (the real CGO version frees C resources).
+func TestCleanup_StubNoOp(t *testing.T) {
+	// Should not panic and completes immediately
+	Cleanup()
+}