Update Rust to 1.97.1

[lukewilliamboswell]

Summary

• pin the repository toolchain to Rust 1.97.1, the latest stable point release
• raise the declared minimum Rust version from 1.83 to 1.97
• apply the formatting produced by rustfmt 1.97.1

Release announcement: https://blog.rust-lang.org/2026/07/16/Rust-1.97.1/

Temporary stack

This branch temporarily includes commit 6f3703d from #458 so the Rust upgrade can be validated with the current Roc compiler. Because #458 comes from a fork, GitHub cannot use its head branch directly as this PR's base. Once #458 merges, this branch can be rebased onto main, leaving only the Rust upgrade diff.

Testing

• cargo fmt --check
• cargo test --locked --all-targets (8 passed)
• cargo build --locked --release
• python3 -m unittest scripts/test_check_release_version.py (3 passed)
• ./scripts/test.py --operation validate
  • rebuilt the x64 musl host with Rust 1.97.1
  • platform tests: 183 passed
  • all example formatting, checks, and tests passed