refactor(arkworks): return SEC1 bytes directly

[wj00243-lgtm]

Summary

• change rust-arkworks sec1_affine to return the 33-byte SEC1 compressed encoding directly
• remove the deprecated helper wrapper while preserving identity encoding internally for challenge preimages
• update SEC1 encoding tests to use the direct byte-array API

Why

Issue #88 notes that rust-arkworks is over-generalized, including the concrete follow-up that affine-to-bytes should return [u8; 33]. This PR keeps the scope narrow and only addresses that API simplification.

Tests

• added sec1_affine_returns_compressed_bytes_directly
• updated test_point_sec1_encoding
• git diff --check passed
• cargo test was not run locally because cargo/rustc is not available in this environment

Closes #88