Feat: add purego ext 6 over koalabear and babybear fields

[yelhousni]

Description

We might need extension 6 over koalabear instead of 4 for higher security. This PR implements purego ext 6 over koala/baby bears.

Type of change

• New feature (non-breaking change which adds functionality)

How has this been tested?

Corresponding tests implemented in e6_test.go

How has this been benchmarked?

Benchmarks in e6_test.go

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

---

Note

Adds pure-Go 𝔽r⁶ extensions (tower E6 and direct E6D) for babybear/koalabear, updates docs/generator, and renames non-residue ops in E2/E4 with full tests/benchmarks.

• Extensions:
  • Add 𝔽r⁶ tower E6 and direct E6D implementations for babybear and koalabear with ops: Add/Sub/Mul/Square/Inverse/Exp/BatchInvertE6/Div/Halve/ButterflyE6.
  • Wire generation: new templates e6.go(.tmpl), e6_direct.go(.tmpl), and tests in generator; enable in generate_extensions.
• API changes (E2/E4):
  • Rename MulByNonResidue → MulByQuadraticNonResidue; remove inverse variant and add MulByCubicNonResidue (where applicable).
  • Update E4 to use MulByQuadraticNonResidue in Mul/Square/Inverse/norm; drop non-residue benchmarks.
  • Minor: SetOne comment clarified.
• Docs:
  • Expand extensions/doc.go to document extension-6 constructions and equivalences for babybear/koalabear; clarify goldilocks extension-2.
• Generator/Tooling:
  • Add goimports step to formatter; register new E6 files in extensions generator.
• Tests/Benchmarks:
  • Add comprehensive property tests and benches for E6/E6D; update E2/E4 tests to new method names and benches accordingly.

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[gbotrel]

lgtm, just add goimports to your flow half of the files are modified / staticcheck doesn't pass because of this :)

[TomWambsgans]

Hello! I am curious, what is the current state of performance of the degree 6 extension for koala-bear (in particular vs degree 5 if you have any number), and which algorithm is the fastest on gnark? (is it Toom3?) Thanks in advance ;)