`provekit/common/src/field`: base-bridge): add a base (`Source`) bridge when base commitment lands

[recmo]

On 2026-07-06 in 05de18e “Merge pull request #458 from worldfnd/sl/goldilocks-field-abstraction”:

base-bridge): add a base (Source) bridge when base commitment lands
V-stage).

    type Sponge: spongefish::DuplexSpongeInterface<U = u8> + Send;

    /// Construct the transcript sponge matching `config`.
    fn transcript_sponge(config: crate::HashConfig) -> Self::Sponge;

    // TODO(base-bridge): add a base (`Source`) bridge when base commitment lands
    // (V-stage).
}

From provekit/common/src/field.rs:47