sync
open override fun syncJsfxRegisteredFolders(completed: (filesWritten: UInt, error: String?) -> Unit)
AppModel::syncJsfxRegisteredFolders(): re-reads every registered JSFX folder and rebuilds its mirror. completed runs on the main thread, with the number of files written and the first error, or null. It completes at once when no folder is registered, or without JSFX.