Function handle_cancel 

async fn handle_cancel<E, C>(
    partial_path: &PathBuf,
    request_id: &str,
    registry: &TransferRegistryHandle,
    lock_registry: &LockRegistry,
    lock_id: &LockId,
    channel: &E,
    log: &NotificationLogHandle,
    os_notifier: &OsNotifier<C>,
) -> Result<(), AppError>
where
    E: EventEmitter,
    C: OsNotifyChannel,

Cancel path: delete .partial, emit Canceled, release lock.