`provekit/common/src/field`: add a base-field (`Source`) byte bridge when one is needed.

[recmo]

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

add a base-field (Source) byte bridge when one is needed.

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

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

    // TODO: add a base-field (`Source`) byte bridge when one is needed.
}

From provekit/common/src/field.rs:53