feat: add portable directory safeguards

internal/platform/files/instance_data_remover_test.go

@@ -81,6 +81,36 @@ func TestInstanceDataRemoverHonorsCanceledContext(t *testing.T) {
 	}
 }
 
+func TestInstanceDataPreparerCreatesAndVerifiesWritableDirectory(t *testing.T) {
+	target := filepath.Join(t.TempDir(), "new-profile")
+	if err := NewInstanceDataPreparer().PrepareInstanceDataDir(context.Background(), target); err != nil {
+		t.Fatal(err)
+	}
+	info, err := os.Stat(target)
+	if err != nil || !info.IsDir() {
+		t.Fatalf("prepared directory = %#v, error = %v", info, err)
+	}
+	probes, err := filepath.Glob(filepath.Join(target, ".chub-write-*"))
+	if err != nil || len(probes) != 0 {
+		t.Fatalf("write probes = %#v, error = %v", probes, err)
+	}
+}
+
+func TestInstanceDataPreparerRejectsUnavailableDirectoryBeforeBrowserLaunch(t *testing.T) {
+	target := filepath.Join(t.TempDir(), "profile")
+	preparer := newInstanceDataPreparer(os.MkdirAll, os.Lstat, func(string, string) (*os.File, error) {
+		return nil, errors.New("denied")
+	}, os.Remove)
+	if err := preparer.PrepareInstanceDataDir(context.Background(), target); !errors.Is(err, ErrInstanceDataDirectoryUnavailable) {
+		t.Fatalf("unwritable directory error = %v", err)
+	}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	if err := NewInstanceDataPreparer().PrepareInstanceDataDir(ctx, filepath.Join(t.TempDir(), "canceled")); !errors.Is(err, context.Canceled) {
+		t.Fatalf("canceled preparation error = %v", err)
+	}
+}
+
 type fakeFileInfo struct {
 	mode os.FileMode
 	sys  any