z0p v0.2.1 — clean rustdoc CI

z0p v0.2.1

Patch release for the v0.2.0 line. No protocol or wire-format changes.

Fixed

• Rustdoc now builds cleanly with -D warnings: the sumcheck module docs
  no longer link to the crate-private verify_final helper.
• tools/check.py now includes the same rustdoc gate used by CI.

Verification

All CI jobs pass: stable fmt/clippy/tests, MSRV 1.74, rustdoc with warnings
denied, and portable SIMD on nightly. The 1M-step AIR benchmark remains at
approximately 1.76 s prove / 2.2 ms verify / 203,553 B.

Full changelog: v0.2.0...v0.2.1