Function describe_columns 

pub fn describe_columns(bytes: &[u8]) -> Vec<String>

Name the structural encoding of each column in a native wire message — the codec explaining its own output, so “which dial won” is legible to a human. A single-column message (int / float / string / bool list, a set, an int-keyed map) yields one name; a record list (T_STRUCTS) yields one "field: encoding" per field. Empty for a shape it does not model (compressed, JSON, a bare scalar, or a malformed frame) — the caller then shows a generic label. It reuses the real decode dispatch to skip each column body, so it can never drift from the format it reports on.