Add cargo audit to CI

[alltheseas]

Cargo Audit Purpose

Feature	Description
Purpose	Audits Rust dependencies for security issues
Checks	Vulnerabilities, yanked crates, unmaintained crates
Database	Uses RustSec Advisory Database
Usage	cargo audit
Install	cargo install cargo-audit
Common in	CI pipelines and security-conscious Rust projects

Summary

• add a cargo-audit job to .github/workflows/rust.yml alongside the existing
  lint/test matrix
• install cargo-audit via taiki-e/install-action and run cargo audit --deny
  warnings so advisories fail the pipeline

Testing

• cargo audit findings Oct 27 2025 run below. Otherwise github CI run will execute the new job
• slab 0.4.10 — RUSTSEC-2025-0047 — Out-of-bounds access in get_disjoint_mut —
  upgrade to ≥0.4.11
  • tracing-subscriber 0.3.19 — RUSTSEC-2025-0055 — Logging user input can
    inject ANSI escape sequences — upgrade to ≥0.3.20
• findings addressed in standalone PR here