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() {}