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

internal/photos/cgo_bridge_test_impl.go

@@ -78,6 +78,10 @@ func (*CgoBridge) Cancel() {
 	C.photos_request_cancel()
 }
 
+func (*CgoBridge) IsCancelled() bool {
+	return C.photos_request_is_cancelled() != 0
+}
+
 func (*CgoBridge) ExportPreview(assetID, outputDir string, targetSize, index int) (ExportResult, error) {
 	cid := C.CString(assetID)
 	defer C.free(unsafe.Pointer(cid))