Make arg_parse outputs transactional

[codeforester]

Summary

• Parse option results into local arrays before publishing caller outputs.
• Preserve caller-owned options and positionals when a late parse error occurs.
• Add regression coverage for unknown-option and missing-value failures after accepted args.

Fixes #145

Validation

• bats lib/bash/arg/tests/lib_arg.bats
• git diff --check
• ./tests/validate.sh