Function local_signature 

pub fn local_signature(rec: &Record) -> LocalSignature

Target-blind structural key for a record: fields sorted by label, each keyed by (label, min, max, shape).

Fields are sorted by label rather than kept in declaration order: validation ignores field order (a Record is a set of labeled fields), so two records that declare the same fields in a different order accept exactly the same documents and MUST land in the same initial partition block – keying by declaration order would incorrectly split them.