feat: add Goldilocks backend + two-field differential fixtures

[BornPsych]

Summary

Adds provekit-backend-goldilocks, a second backend that instantiates the field-generic spine over the ~192-bit Goldilocks cubic extension Field64_3, and restructures provekit-fixtures so the prove→verify and soundness suites run over both bn254 and goldilocks from shared, field-generic test bodies. bn254 is unchanged.

This is the backend half of the field-generic work: #458/#459 made the spine generic over <P: ProofField> and added the scheme builder + fixtures; this PR shows a second, structurally different field plugs in with no spine edits.

Why

The spine was made field-generic precisely so a non-bn254 field could be added as a backend crate plus an instantiation, with no changes to common / prover / verifier. Goldilocks is the validation field: a 64-bit prime whose cubic extension Field64_3 is large enough (~192 bits) to draw Fiat-Shamir challenges from. This PR is the proof that the seam holds — the backend is a marker type implementing ProofField / FieldHash plus a register() call, nothing more. (Pre-EF/BF the embedding is Identity<Field64_3>, so base == ext for now; the base≠ext split lands once zkWHIR supports it.)

What changed

• provekit-backend-goldilocks (new): GoldilocksField with Embedding = Identity<Field64_3>; a FieldHash impl with a Sha256 default hash, 24-byte little-endian canonical bytes, digest-spread hashing, and a transcript sponge; register() installs the Field64_3 NTT engine in whir's global registry. Mirrors the provekit-backend-bn254 layout.
• provekit-fixtures: the bn254-only roundtrip.rs / soundness.rs are replaced by tests/shared/mod.rs — field-generic <P: FieldHash> test bodies plus roundtrip_suite! / soundness_suite! macros — and four thin per-field instantiation files (bn254_*, goldilocks_*). The same assertions now run over both fields.
• Workspace: registers the new crate as a member + workspace dep, and adds it as a fixtures dev-dep.

Verification

• cargo build --workspace green.
• provekit-fixtures: bn254 6 roundtrip + 4 soundness, goldilocks 6 roundtrip + 4 soundness — all pass; 2 benches ignored.
• provekit-backend-goldilocks unit tests (byte roundtrip) pass.

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Stacks on #459 (open); until that merges, the commit list above includes #459's commits — the Goldilocks-specific changes are the final 2 commits.

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