style: add rustfmt.toml, apply cargo fmt, and add justfile

[arhtur007]

Description

• Establish formatting baseline with rustfmt defaults so future changes don't mix logic and formatting diffs.
• Add justfile with just fmt (auto-format) and just lint (fmt check + clippy) commands for local development.
• No CI workflow added — GitHub Actions runner minutes are limited for open source projects, and a PR-only check would consume quota quickly. Contributors are encouraged to run just lint locally before submitting PRs.

Contributing Note

Consider adding the following to the contributing guide:

• Run just fmt to auto-format code before committing.
• Run just lint to verify formatting and catch common issues (runs cargo fmt --check + cargo clippy).