Function scan_label_chars 

pub(crate) fn scan_label_chars(bytes: &[u8], from: usize) -> usize

Returns the byte offset just past the longest label-identifier run that starts at from in bytes. Empty (caller should detect via id_end == from) if the first byte is not a valid identifier char.

The accepted alphabet matches manifest §3.3 examples: [A-Za-z0-9_:.-]. Critically : is included so fig:wells and eq:bayes round-trip.