Update Rust

[stepchowfun]

This updates the pinned Rust toolchain:

• Rust: 1.98.1 → 1.99.0 (and the nightly used for Rustfmt: 2026-09-16 → 2026-10-01). Formatting with the new nightly produced no changes.

All crate dependencies are already at their latest versions, and cargo update found no lockfile changes.

Rust 1.99.0 adds the pedantic clippy::assert_is_empty lint, so tests that used assert!(check(...).is_empty()) now use assert_eq!(check(...), Vec::<String>::new()), which also shows the unexpected errors on failure.

The copyright year in LICENSE.md is already current, and all GitHub Actions are already at their latest versions.

Status: Ready