Add standalone base-bash launcher

[codeforester]

Summary

• add bin/base-bash, a standalone launcher for scripts using #!/usr/bin/env base-bash
• preload lib/bash/std/lib_std.sh, preserve wrapper flag handling, and call a script-defined main
• add import_base_bash_lib for package-relative companion library imports
• document source-checkout and Homebrew-style launcher usage

Fixes #97.

Validation

• bats tests/launcher.bats
• shellcheck --severity=error bin/base-bash tests/launcher.bats tests/validate.sh
• ./tests/validate.sh

Release note

The Homebrew tap should install bin/base-bash when the release containing this change is published.