Function salt_strength 

pub fn salt_strength(salt: Option<&str>) -> SaltStrength

Pure, deterministic salt-strength classifier shared by the sha2_salted UDF and the startup warning. Returns a verdict class WITHOUT retaining or exposing the value.

A salt is SaltStrength::Usable only when, after trimming, it is non-empty, at least MIN_SALT_LEN characters, not an obvious placeholder, and not a run of a single repeated character.