Reject option tokens as arg values

[codeforester]

Summary

• Treat a value option followed by a registered option token as a missing value.
• Document the ambiguity rule for option-like values.
• Add BATS coverage for the registered-option case.

Validation

• shellcheck --severity=error lib/bash/arg/lib_arg.sh lib/bash/arg/tests/lib_arg.bats
• bats lib/bash/arg/tests/lib_arg.bats
• ./tests/validate.sh

Fixes #77