`provekit/common/src/whir_r1cs`: make the witness commitment hiding for full witness ZK.

[recmo]

On 2026-07-13 in 61744fa “Merge pull request #470 from worldfnd/feat/base-field-witness-commit”:

make the witness commitment hiding for full witness ZK.

    pub num_challenges:    usize,
    pub challenge_offsets: Vec<usize>,
    pub has_public_inputs: bool,
    /// Base-field witness commitment. Non-hiding — WHIR openings leak witness
    /// values, so this provides sumcheck ZK only, not witness ZK.
    /// TODO: make the witness commitment hiding for full witness ZK.
    pub whir_witness:      GenericWhirConfig<P::Embedding>,
    /// Separate ext-field commitment to the Spartan blinding polynomial `g`;
    /// masking the ext-valued sumcheck rounds needs ext randomness, so `g`
    /// cannot ride on the base witness commitment.
    pub whir_blinding:     GenericWhirConfig<Identity<Ext<P>>>,

From provekit/common/src/whir_r1cs.rs:85