Remove atty in favor of std terminal checks and expand test coverage

[stephenlclarke]

• Removed the unmaintained atty dependency and switched terminal detection to the standard std::io::IsTerminal.
• Expanded display rendering coverage with synthetic schema tests (message rendering, msg-type enum selection, layout caching, and layout computation).
• Added CLI helper tests for FIX version normalization, dictionary key/source selection, and service pack handling.
• Added summary helpers coverage for settlement date selection, timestamp date extraction, and day-diff edge cases.