Use in-place string transformations

[codeforester]

Summary

Change lib/bash/str/lib_str.sh so string transformation helpers mutate named variables in place instead of printing transformed values for command substitution.

Motivation

The current str_lower, str_upper, and trim helpers encourage command substitution such as value=$(str_trim "$value"), which creates a subshell. Since these helpers are shell-library functions, the preferred API should avoid subshells and use validated variable names with printf -v.

Scope

• Add a public stdlib assertion for valid Bash variable names.
• Change string transformation helpers to accept variable names and mutate values in place.
• Keep predicate helpers value-based because they do not transform data.
• Keep split/join/member helpers result-variable or predicate style, but use the public stdlib assertion instead of the private validator.
• Update README and BATS coverage.

Validation

• Focused BATS for stdlib and string helpers.
• ./tests/validate.sh.
• git diff --check.