fix: poseidon2 compression didn't include feed-forward

[ivokub]

Description

The compression function of Poseidon2 permutation differs between gnark and gnark-crypto. gnark-crypto includes right side feed-forward in the result but gnark not. This made the results differ.

Also added example test for non-default parameters to see that instance generation matches between gnark and gnark-crypto

Strangely - this was tested before but CI didn't fail? I don't see any obvious reasons, @gbotrel maybe you have ideas? I think it is somehow related which curves we test - currently the test was only for BLS12-377 as we don't have default parameters for other curves yet, but I think that for some test configuration it completely discarded testing.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

Added new test. Current test works locally. Strangely the test should have had failed in CI before, but didn't?

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

[Copilot]

Pull Request Overview

This PR fixes the Poseidon2 compression function by including the missing feed-forward component, ensuring that the behavior between gnark and gnark-crypto is consistent. It also adds an example test for using non-default parameters to validate instance generation between the two libraries.

• Fixed compression logic in Poseidon2 to add the feed-forward value.
• Added a custom parameter test to validate implementation consistency.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
std/permutation/poseidon2/poseidon2.go	Updated Compress to add the feed-forward during compression.
std/hash/poseidon2/poseidon2_test.go	Added a test (TestPoseidon2Custom) to verify non-default parameter behavior.

[Tabaie]

#1442 covers this.