ci: add line length lint step using nightly cargo fmt

[antiguru]

Summary

• Add a new CI lint step that runs cargo +nightly fmt -- --check --config error_on_line_overflow=true to catch overly long lines in Rust source files.

Test plan

• Verify the new lint-line-length step appears in CI and runs successfully
• Confirm it catches lines exceeding the configured max width

🤖 Generated with Claude Code

[github-actions]

Pre-merge checklist

• The PR title is descriptive and will make sense in the git log.
• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
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