fix(backup): privately stage restore archive

tools/tht/internal/backup/preflight_test.go

@@ -455,6 +455,37 @@ func TestPreflightStagesArchiveIntoImmutablePrivateBytes(t *testing.T) {
 	}
 }
 
+func TestStageArchiveCleansPrivateFileAfterStreamingFailure(t *testing.T) {
+	installation := preflightTestInstallation(t)
+	if err := os.MkdirAll(installation.ControlDirectory(), 0o700); err != nil {
+		t.Fatal(err)
+	}
+	archive := filepath.Join(t.TempDir(), "checked.zip")
+	writePreflightArchive(t, archive, preflightArchiveSpec{
+		entries: []preflightArchiveEntry{{path: "configuration/operator.env", body: []byte("before")}},
+	})
+
+	result, err := Preflight(context.Background(), installation, PreflightRequest{Archive: archive, Confirm: true}, permissivePreflightDependencies())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer result.CloseArchive()
+	result.freeBytes = func(string) (uint64, error) {
+		result.archive.digest = "after-preflight-mismatch"
+		return 1024, nil
+	}
+	if _, err := result.StageArchive(context.Background()); err == nil || !strings.Contains(err.Error(), "changed") {
+		t.Fatalf("StageArchive() error = %v, want streaming size refusal", err)
+	}
+	entries, err := os.ReadDir(result.stagingRoot)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(entries) != 0 {
+		t.Fatalf("private staging leftovers = %v, want none", entries)
+	}
+}
+
 func TestPreflightStagingRejectsInPlaceArchiveHashMutation(t *testing.T) {
 	installation := preflightTestInstallation(t)
 	archive := filepath.Join(t.TempDir(), "checked.zip")