Harden string helper contracts

[codeforester]

Summary

• Add arity checks to string predicate helpers.
• Avoid str_split result collisions with an array named fields.
• Document predicate arity and add regression coverage.

Validation

• shellcheck --severity=error lib/bash/str/lib_str.sh lib/bash/str/tests/lib_str.bats
• bats lib/bash/str/tests/lib_str.bats
• ./tests/validate.sh

Fixes #79