Add Poseidon circomlib compatibility tests

[keilogic]

Summary

• add dev-only circomlib reference wrappers for Poseidon widths 1, 2, 4, and 16
• add a Mocha compatibility test that compares opencircom witness outputs against circomlib for fixed vectors
• document the Poseidon compatibility check in the README

Verification

• node --check test/poseidon_compat_test.js
• git diff --check

Not run locally:

• npm run compile:test / npm test because this Windows environment does not have circom 2.x on PATH. Running the compile script through Git Bash reaches the project check and stops with: Error: circom not found on PATH. Install circom 2.x first.

Closes #19.

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