CI: add cargo doc and cargo audit

[kiyeonjeon21]

Current CI runs clippy, fmt check, and cargo test. Missing:

• `cargo doc --no-deps --document-private-items` — catches broken doc links and missing docs
• `cargo audit` (via rustsec/audit-check) — flags known vulnerabilities in dependencies

Both are cheap to run and prevent regressions.

File: .github/workflows/ci.yml