fix(circom): reuse compressed public key in V1 challenge - #153

Fixes #14

Problem

The V1 circuit compressed the public key twice: once for the hash-to-curve preimage and once again while building the V1 challenge hash preimage.

Solution

This exposes the already computed compressed public key from check_ec_equations and reuses it in the V1 challenge SHA-256 helper. The challenge preimage bytes stay the same, but the circuit avoids a duplicate public-key compression component.

Tests

• Added a focused v1_challenge_sha_256_test.circom test circuit.
• Extended sha256Circuit.test.ts to verify the V1 challenge hash accepts the reused compressed public key bytes and still matches the existing digest.
• Ran git diff --check.

I could not run the full local circuit suite in this environment because pnpm install fails while fetching https://gitpkg.now.sh/geometryresearch/secp256k1_hash_to_curve/circuits?main with HTTP 402. The local Node version is also v24.15.0 while this package declares >=16 <19.