feat(builtins): implement expr builtin

[chaliy]

Summary

• Add expr builtin supporting arithmetic (+, -, *, /, %), comparison (=, !=, <, >, <=, >=), string operations (length, substr, index, match), and pattern matching (: operator)
• Handle exit codes per POSIX: 0 for truthy results, 1 for zero/empty results, 2 for errors
• 13 spec tests covering arithmetic, string ops, comparisons, pattern matching, error cases, and exit code semantics

Test plan

• cargo clippy --all-targets --all-features -- -D warnings passes
• cargo test --all-features --test spec_tests bash_spec_tests passes
• All 13 expr spec tests pass
• Spec counts updated (Bash 971, Total 1389)