Function transposed_constant_terms 

pub(crate) fn transposed_constant_terms<S: PixSpec>(
    commitments: Vec<SsaPartCommitment<S>>,
) -> HashMap<CoefficientIndex, Vec<(PolynomialIndex, PixGroupRepr<S>)>>

Lays the per-polynomial constant-term commitments out in the coefficient-major form the wire messages expect.

The result always holds exactly one key, CONSTANT_TERM_COEFFICIENT. The map shape is kept rather than flattened to a plain Vec because it is what SsaClientCommitmentMessage splits into packets, and the wire format still admits higher coefficient indices even though PIX no longer produces any.