Function observe_var_len_public_inputs 

pub fn observe_var_len_public_inputs<C: CanObserve<Felt>>(
    challenger: &mut C,
    var_len_public_inputs: &[&[Felt]],
    message_widths: &[usize],
)

Absorbs variable-length public inputs into the challenger.

Each VLPI group is a flat slice of fixed-width messages. message_widths[i] gives the width of each message in group i. Every message is zero-padded to the next multiple of SPONGE_RATE and reversed before observation, matching the layout the MASM recursive verifier’s mem_stream + horner_eval_base expects.