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

bridge/photokit_bridge.h

@@ -28,6 +28,8 @@ char *photos_list_tree_json(void);
 
 void photos_request_cancel(void);
 
+int photos_request_is_cancelled(void);
+
 void photos_free_string(char *value);
 
 #ifdef __cplusplus

bridge/photokit_bridge.m

@@ -415,3 +415,7 @@ void photos_free_string(char *value) {
 void photos_request_cancel(void) {
     photos_cancelled = 1;
 }
+
+int photos_request_is_cancelled(void) {
+    return photos_cancelled;
+}

bridge/photokit_bridge_stub.c

@@ -74,6 +74,10 @@ void photos_request_cancel(void) {
     stub_cancelled = 1;
 }
 
+int photos_request_is_cancelled(void) {
+    return stub_cancelled;
+}
+
 void photos_test_set_export_preview_json(const char *json) {
     stub_export_preview_json = json;
 }