[erabinov]

CRYP-663

Summary

• Add portable GF(2^8) and GF(2^128) GHASH arithmetic to SLOP.
• Add Criterion tests for four field operations with iteration-based inputs.
• Limit CI work for SLOP-only changes and CPU-only changes.

Motivation

This change starts the field migration from Flock into SP1. It provides a portable base before SIMD work.

Validation

• cargo test -p slop-binary-fields
• cargo check -p slop-binary-fields --benches
• cargo clippy -p slop-binary-fields --all-targets -- -D warnings
• cargo fmt --all -- --check
• Shell syntax and workflow YAML checks

Linear Reference

CRYP-656