Function parse_facs_string 

pub fn parse_facs_string(s: &str) -> FacsState

Parse a FACS string such as "AU1+AU6+AU12" or "AU1A+AU12E".

Tokens are separated by '+'. Each token must start with "AU" (case- insensitive) followed by a decimal number and an optional intensity letter (A–E). Missing intensity defaults to 1.0.