fix(backup): use platform private staging controls

tools/tht/internal/backup/preflight_windows_test.go

@@ -0,0 +1,40 @@
+//go:build windows
+
+package backup
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/aritmolab/thothii/tools/tht/internal/safeio"
+)
+
+func TestStageArchiveProtectsWindowsStagingDirectoriesWithOwnerOnlyACLs(t *testing.T) {
+	installation := preflightTestInstallation(t)
+	if err := os.MkdirAll(installation.ControlDirectory(), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	archive := filepath.Join(t.TempDir(), "valid.zip")
+	writePreflightArchive(t, archive, preflightArchiveSpec{
+		entries: []preflightArchiveEntry{{path: "configuration/operator.env", body: []byte("safe")}},
+	})
+	result, err := Preflight(context.Background(), installation, PreflightRequest{Archive: archive, Confirm: true}, permissivePreflightDependencies())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer result.CloseArchive()
+
+	staged, err := result.StageArchive(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer staged.Close()
+	if err := safeio.ValidatePrivateDirectory(result.stagingRoot); err != nil {
+		t.Fatalf("staging root ACL = %v, want owner-only", err)
+	}
+	if err := safeio.ValidatePrivateDirectory(staged.directory); err != nil {
+		t.Fatalf("staged archive directory ACL = %v, want owner-only", err)
+	}
+}