[PROPOSAL] Add Rust/Solana export commands for Groth16

[vitorpy]

Summary

Solana's groth16-solana crate, as well as most Solana ZK applications, use snarkjs for generating proofs. I'm adding, mostly verbatim from groth16-solana JS scripts, native methods to the snarkjs CLI to export Solana-compatible artifacts.

This is a proposal - I'm happy and more than open to take feedback and rework this in a way that's amenable to snarkjs maintainers.

New Commands

• snarkjs zkey export rustverifier [circuit.zkey] [verifier.rs] (alias: zkerv)

  • Exports Groth16 verification key as a self-contained Rust file with struct definition

• snarkjs zkey export rustcalldata [public.json] [proof.json] (alias: zkerc)

  • Exports proof elements and public inputs as Rust constants

Implementation Details

The byte transformations follow the conventions established by groth16-solana for compatibility with Solana's alt_bn128 precompiles:

• G1 points: Each coordinate → little-endian 32 bytes → reverse → 64 bytes total
• G2 points: Concatenate coordinate pair (64 bytes) → reverse all → 128 bytes total
• Scalars: Little-endian 32 bytes → reverse → 32 bytes

Test Plan

• Verify rustverifier output matches existing groth16-solana parse_vk_to_rust.js output
• Verify generated Rust code compiles
• Test with groth16-solana verifier
• All existing tests pass (49/49)