Use in-place string transformations

[codeforester]

Summary

• Add public assert_variable_name to stdlib for helpers that accept Bash variable names.
• Change string case and trim helpers to mutate named variables in place instead of printing values for command substitution.
• Update string validation, README guidance, changelog, and BATS coverage for the no-subshell API.

Validation

• RED: bats lib/bash/std/tests/lib_std.bats lib/bash/str/tests/lib_str.bats failed on missing assert_variable_name and old stdout string transforms.
• bats lib/bash/std/tests/lib_std.bats lib/bash/str/tests/lib_str.bats
• ./tests/validate.sh
• git diff --check

Fixes #64