future (.method = "future")
Sets a future::multisession plan (portable across OSes) and registers a foreach
backend via doFuture::registerDoFuture(). This avoids the tune foreach/future warning.
 
parallel (.method = "parallel")
parallel::makeCluster(...)  2) doParallel::registerDoParallel(cl)
 
Set .libPaths() on workers; optional clusterExport and package loads.
 
spark (.method = "spark")
Requires sparklyr::spark_connect(); registers foreach via sparklyr::registerDoSpark(...).