refactor(GKR): Remove the Z/pZ assumption

[Tabaie]

This PR refactors the GKR codebase, in particular gate testing infrastructure and code generation, so that they accommodate extension fields.

NOTE: Pinning an unmerged gnark-crypto. Only to be merged after that.

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Note

High Risk
Changes Fiat–Shamir binding/challenge derivation and gate compilation over all GKR curves, which can alter proof transcripts and break compatibility with prior proofs if encodings differ.

Overview
This PR drops the implicit “everything is ℤ/pℤ” assumption in GKR by threading a gkrcore.Field description (prime field via PrimeField, extensions via minimal polynomial) through gate compilation, gate metadata discovery (degree / solvable variable), circuit compile APIs, and test caches. Gate testing now runs over both BN254 and a sample extension field so metadata stays consistent outside prime fields.

Fiat–Shamir is centralized: per-curve transcript copies are removed in favor of gkrcore.HashTranscript, which binds elements with Marshal() and derives challenges via Challenge() (replacing getChallenge / Bytes()). Prove/verify and sumcheck paths across curves are updated accordingly.

Supporting cleanup includes renaming internal/small_rational → internal/rational, exposing Field / PrimeField on gkrapi.Compile, updating codegen templates, refreshing GKR benchmarks to Poseidon2, tightening test-vector parsing (big.Rat, Marshal in fake hashes), fixing Groth16 test proof padding for empty commitments, and bumping gnark-crypto.

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[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	golang/​github.com/​consensys/​gnark-crypto@​v0.21.1-0.20260913224755-1409fe592052 ⏵ v0.21.1-0.20261002031104-9e7ec3b0d8ca	+1

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