ci: modernize Rust quality checks

[lu-zero]

Summary

• replace the obsolete actions/checkout@v2 with @v7
• install stable Rust with rustfmt and clippy
• run checks for pushes, pull requests, and manual dispatches
• check all targets/features, run tests and doctests, and enforce warning-free Clippy
• fix the two existing strict-Clippy findings: an unnecessary mut and an item declared after the test module

Validation

• cargo fmt --all -- --check
• cargo check --workspace --all-targets --all-features
• cargo test --workspace --all-targets --all-features
• cargo test --workspace --all-features --doc
• cargo clippy --workspace --all-targets --all-features -- -D warnings