Validate variable-name helper arguments

[codeforester]

Summary

• Centralize Bash variable-name validation in stdlib.
• Validate stdlib assertion/result helpers before indirect expansion or printf -v.
• Validate git_get_current_branch result variable names before assignment.
• Add tests covering stdlib and git invalid-name diagnostics without leaking passed values.

Validation

• RED: focused invalid-name BATS filters failed before the fix with raw/unclear behavior.
• GREEN: focused invalid-name BATS filters pass.
• bats lib/bash/std/tests/lib_std.bats
• bats lib/bash/git/tests/lib_git.bats
• shellcheck --severity=error lib/bash/std/lib_std.sh lib/bash/std/tests/lib_std.bats lib/bash/git/lib_git.sh lib/bash/git/tests/lib_git.bats
• ./tests/validate.sh
• git diff --check

Fixes #14