Feat: Add Canonical Round Constants for Poseidon2 KoalaBear Spec

[tcoratger]

Context

In #3, a discussion was raised regarding the generation of round constants (#3 (comment)). The current spec uses a simplified, non-compliant function (_generate_spec_test_round_constants) that generates sequential integers ([0, 1, 2, ...]).

This approach was intentionally chosen as a temporary measure to simplify initial implementation and allow for direct comparison with a reference Rust implementation without needing to hardcode a large, arbitrary array of values.

The Problem

While useful for testing the algebraic structure, using simple sequential integers is not representative of a production-grade cryptographic primitive.

The current function is explicitly named to reflect its non-compliant, test-only nature, but the long-term goal for a canonical specification should be to use fixed, pseudo-random constants.

Proposed Solution

To align the KoalaBear Poseidon2 spec with best practices and other reference implementations, we should replace the test generation function with a pre-computed, hardcoded array of canonical round constants.

This would follow the precedent set by other fields, such as BabyBear, which uses a set of hardcoded constants for performance and security reasons.
Reference: Plonky3 BabyBear Poseidon2 Constants

[khovratovich]

KoalaBear. Width=16, RF=8, RP=20
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1131403964, 1724233597, 1086876318, 669451611, 1829624280, 2119538869, 441255155, 1580936135, 1396398895, 1043570981, 1716351438, 942566442, 616885102, 334644983, 132306927]

[tcoratger]

Thank you very much @khovratovich, I just integrated them into this PR #6.

When you have 5 minutes, could you take a look?

Since there are too many constants in the table you put here, I imagine they need to be split arbitrarily, as I did in the PR. Tell me if this split is correct :)