ci: add cargo fmt check to CI

[ElFantasma]

Add a cargo fmt --all -- --check step to the CI workflow.

This was deferred to avoid formatting conflicts with PR #153. After that PR merges:

1. Format the codebase with cargo fmt --all
2. Add a fmt job to .github/workflows/ci.yml

The workflow change and formatting fixes are already prepared on branch ci/cargo-fmt (stashed).

[ElFantasma]

Done — merged as PR #158.