Function decode_operand 

pub fn decode_operand<const N: usize>(
    operand: &KappaLabel<N>,
) -> Result<(&str, Vec<u8>), CompositionFailure>

Decode a κ-label’s raw digest bytes (the lowercase-hex body after the σ-axis prefix). Returns the σ-axis name (the prefix without trailing :) and the raw digest bytes (32 or 64 bytes).