Granary_parallel.ParallelSourcePortable multi-core gate around OCaml 5 domains.
Lets call sites offer a parallel implementation that is only taken when the runtime actually has more than one domain available, falling back to a sequential equivalent on monocore targets (notably Solo5/Mirage, where a second domain cannot be spawned).
true when the runtime recommends more than one domain — i.e. a multi-core Unix host. Always false on monocore Solo5/Mirage, where Domain.spawn cannot create a second domain.
run ~parallel ~sequential evaluates parallel () when available, otherwise sequential (). On Solo5 it always takes sequential, so a Domain.spawn placed inside parallel is never reached. The two thunks must be observably equivalent.