chore(corelib): rename falcon512poseidon2 to falcon512_poseidon2

[KitaroMoha563]

Renames miden::core::crypto::dsa::falcon512poseidon2 to falcon512_poseidon2 to match the snake_case convention used in miden_crypto, closes #2740.

[github-actions]

Automated check (CONTRIBUTING.md)

Findings:

• Ensure issue Consider renaming dsa::falcon512poseidon2 to falcon512_poseidon2 #2740 is assigned to the PR author.
• Add a short Rationale explaining why the change is needed.

Recommendations:

• Consider adding a Test plan or clear review steps.

Next steps:

• Ensure issue Consider renaming dsa::falcon512poseidon2 to falcon512_poseidon2 #2740 is assigned to you.
• See CONTRIBUTING.md for expectations.
• If this is a false positive, comment: /quality-review.