feat(bash): implement select construct

[chaliy]

Summary

• Implement select var in list; do body; done construct
• Parse into SelectCommand AST node (similar to ForCommand)
• Execute by reading from pipeline_stdin, printing numbered menu to stderr
• Sets variable to selected item, REPLY to raw input
• Matches bash EOF behavior: prints newline to stdout and exits with code 1
• 5 spec tests: basic selection, REPLY variable, invalid input, multiple iterations, EOF exit

Test plan

• cargo test --all-features passes
• cargo clippy clean
• cargo fmt --check clean
• bash_comparison_tests 789/789 match real bash
• All 864 bash spec tests pass (859 pass + 5 skip)