Function make_id 

pub fn make_id(ordinal: usize, hash: &str) -> String

Build a Reduction::id: a zero-padded 4-digit ordinal plus the first 4 hex characters of a content hash (D2), e.g. "r0042-9f3c".

ordinal is the reduction’s position among reductions applied to this session; it wraps decoratively past 9999 (id uniqueness within a session still holds in practice because the hash prefix disambiguates, and no real session approaches that many reductions).