Function ics_fold 

pub fn ics_fold(line: &str) -> String

RFC 5545 §3.1 folding: 75 octets for the first line, 74 for each continuation, which begins with a space.

Octets, not characters — the same trap the TypeScript twin documents. A name with an accent or an emoji is several bytes per character, and folding by character count yields a line that is legal by one measure and illegal by the one a parser applies.