Function parallel_io 

pub fn parallel_io() -> bool

Whether parallel file I/O pays on this machine: yes where I/O latency dominates (Linux: ext4/WSL2 measured 2x on a wiped vendor/ and on a cold classmap scan), no where the page cache is the bottleneck (APFS: parallel reads 1.3-4x slower, DECISIONS.md M5). VIVACITY_PARALLEL_IO (0/1) overrides the default.