chore: deny std imports when core is available

[3Hren]

Summary

• pin the CI Clippy toolchain to 1.97.1, matching the last green main run and avoiding unreviewed lint drift from floating stable
• deny Clippy's std_instead_of_core lint at the package level
• use core::fmt::Write in the MAC-address benchmark

The lint was verified before fixing the existing import: the all-target Clippy command rejected use std::fmt::Write with clippy::std_instead_of_core.

Testing

• cargo +1.97.1 clippy --all-targets -- -D warnings
• cargo test
• cargo check
• cargo +1.88.0 check --lib --locked
• cargo +nightly fmt --check
• cargo build --lib --target thumbv7em-none-eabihf