Function convfloat_underscore 

pub fn convfloat_underscore(dval: f64, underscore: i32) -> String

!!! WARNING: RUST-ONLY HELPER !!! No C counterpart: C has convbase_underscore() (Src/params.c:5646) for integers but no underscore-grouping variant of convfloat(); the float path in C never groups digits. This helper applies the same grouping to a formatted float for zshrs’s typeset -F/-E underscore support. Format float with underscores