Type Alias SaltFn 

pub type SaltFn = fn() -> [u8; 16];

Function pointer that produces 16 cryptographically random bytes. Like ClockFn, the engine is no_std and can’t reach for /dev/urandom itself — host (spg-server) injects an OS-backed source. None means SQL-driven CREATE USER falls back to a deterministic salt derived from the username (acceptable in tests; the server always installs a real RNG so production paths never see this).