v0.2.4: stop export loop on Ctrl+C instead of flooding failures

- Add IsCancelled() to Bridge interface
- Check bridge.IsCancelled() before each export in serial/parallel/backupTree
- Parallel workers mark remaining slots as 'cancelled' instead of exporting
- Add photos_request_is_cancelled to ObjC and C stub

cmd/photoscli/main_test.go

@@ -50,6 +50,7 @@ func (m *mockBridge) ExportOriginal(assetID, out string, index int) (photos.Expo
 	return photos.ExportResult{Filename: "test.jpg", Size: 2048, Cloud: "cloud"}, nil
 }
 func (m *mockBridge) Cancel() { m.cancelled = true }
+func (m *mockBridge) IsCancelled() bool { return m.cancelled }
 
 func runWith(args []string, b photos.Bridge) (string, string, int) {
 	var out, err bytes.Buffer