refactor: genericize the WHIR-R1CS scheme builder + add provekit-fixtures

[BornPsych]

Summary

Moves WHIR-R1CS scheme construction into the field-generic spine. The scheme parameters (witness commitment domain size, sumcheck rounds, blinding room, zkWHIR config) are now derived generically over the proof field in provekit-common, instead of living in r1cs-compiler welded to bn254's FieldElement. The only genuinely bn254-specific constructor — building a scheme from a Mavros artifact — stays in r1cs-compiler as a thin adapter. bn254 parameter selection is unchanged.

Also adds a new provekit-fixtures crate: a field-generic prove→verify harness plus soundness, roundtrip, and proving-time benchmark tests that exercise the spine on synthetic R1CS instances, independently of any frontend.

This builds on the field-generic spine (<P: ProofField>, #458). Since that work is not yet on main, this PR's commit list includes it; the changes specific to this PR are the final two commits.

Why

#458 made the proof types generic over the field, but scheme construction still lived in r1cs-compiler and took R1CS<FieldElement> / returned a bn254 WhirZkConfig. A Goldilocks backend has to build a scheme without depending on the Noir/mavros compiler (the goldilocks build excludes r1cs-compiler entirely). So the field-generic half of the builder has to move down into the spine, next to the WhirR1CSScheme<P> type it constructs, leaving only the field-specific Mavros adapter behind.

The fixtures crate exists because the generic spine needs coverage that does not route through the bn254 Noir frontend — building R1CS instances with the R1CS API alone lets the same prove→verify path run over any FieldHash proof field once a second backend lands.

What changed

• provekit-common: WhirR1CSScheme<P> gains field-generic constructors new_for_r1cs / new_from_dimensions / new_whir_zk_config_for_size, derived purely from R1CS dimensions and the committed/extension fields of P. The MIN_WHIR_NUM_VARIABLES / MIN_SUMCHECK_NUM_VARIABLES floors move here too. For bn254 (Identity<Fr>, base == ext) the parameters monomorphize to exactly what shipped before.
• provekit-r1cs-compiler: trimmed to a bn254-only MavrosSchemeBuilder adapter. Mavros artifacts are bn254-specific, so this can't move to the spine; it just forwards the instance's dimensions into the generic new_from_dimensions. Call sites spell the field explicitly (WhirR1CSScheme::<Bn254Field>::…).
• provekit-prover: comment cleanup only — the base→ext witness lift is a no-op under any Identity embedding (bn254 and pre-v3 goldilocks), not a bn254-specific fact.
• provekit-fixtures (new): field-generic R1CS builders + a prove→verify harness, pinned to Sha256 so a roundtrip needs no field-specific hash-engine setup. Tests cover R1CS satisfaction, public-input binding (N=1 and N=2), instance binding, scale boundaries, and a small seed fuzz, plus #[ignore]d proving-time benches.

Scope

The fixtures are functional only — they assert that honest proofs verify and that malformed witnesses / public inputs are rejected. They deliberately do not assert bn254 byte-identity; that equivalence is owned by the field-generic spine's regression gate, and the exact m / m_0 / security-level parameter regression stays in the r1cs-compiler builder tests.

Verification

• cargo build / cargo test green over common, r1cs-compiler, prover, and fixtures.
• provekit-fixtures: 6 roundtrip + 4 soundness pass; 2 benches ignored.
• Scaled dual-commit benchmark path runs end-to-end without panic.

Commits

• refactor: genericize the WHIR-R1CS scheme builder in the spine
• test: add provekit-fixtures field-generic prove/verify harness

[vercel]

@BornPsych is attempting to deploy a commit to the World Foundation Team on Vercel.

A member of the Team first needs to authorize it.

[shreyas-londhe]

Thanks for the PR @BornPsych. Clean continuation of the field-generic work. The scheme-builder move keeps bn254 parameter selection identical, splitting the generic constructors from the Mavros adapter is the right line to draw, and the fixtures finally give the spine real frontend-independent coverage with negative tests that actually reject. Left two minor nits inline; neither blocks.