feat: add bn254 poseidon2 constants and support t=4/8/12/16

[devon1209]

Description

Summary of changes:

• add bn254 Poseidon2 constants (round keys + internal diagonal) for t=4/8/12/16 and load them during parameter initialization
• implement external matrix for t=4k using M4 + block-circulant reduction; implement internal matrix using diagM1 for t>=4
• expand supported widths and add parameter validation tests (unskip external matrix test)

Motivation and context:

• enables standard Poseidon2 parameter sets beyond t=2/3 on bn254, matching spec/reference constants and unlocking t=4k permutation widths

Fixes #782

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How has this been tested?

• go test ./ecc/bn254/fr/poseidon2

How has this been benchmarked?

• Not run (not requested)

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

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Note

Medium Risk
Changes core cryptographic permutation parameters and linear-layer math for new widths, so any constant/matrix mismatch would silently break hash compatibility even though validation panics catch some misconfigurations.

Overview
Adds bn254 Poseidon2 precomputed constant parameter sets for widths t=4/8/12/16 (round keys plus internal matrix diagonal), and updates NewParameters to load these constants (with rf/rp validation) instead of deriving keys from a seed.

Extends the permutation to support t=4k by implementing the external linear layer via M4 chunk multiplication plus a block-circulant reduction, and the internal linear layer via the new DiagM1 diagonal; also tightens constructor validation and restricts NewPermutationWithSeed to t=2,3.

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[yelhousni]

The implementation correctly follows the Poseidon2 specification and the mathematical constructions are verified. The code is clean and well-tested for the new functionality. Maybe, consider adding full permutation test vectors for the new widths. Other than that good to merge for me 👍

[devon1209]

I updated the generator templates for the bn254 changes. Could you re-run the workflow?