Fix parallel and compgen issue on NixOS

[Chemaclass]

📚 Description

CC @drupol

🔖 Changes

• Temporary files now respect $TMPDIR, ensuring writable directories during Nix builds
  • Parallel test directories likewise use $TMPDIR for compatibility with sandboxed environments
  • Test targets explicitly run bashunit with bash, avoiding shell mismatch issues during package checks
• Replaced the reliance on compgen with a safer globbing approach so parallel result aggregation works even when programmable completion isn’t available
  • Updated math::calculate to fall back to native shell arithmetic when bc or awk missing, avoiding precision loss

✅ To-do list

• I updated the CHANGELOG.md to reflect the new feature or fix