Adopt Apache-2.0 license and refresh README

[codeforester]

Summary

• Change base-bash-libs from AGPL-3.0-or-later to Apache-2.0 and add NOTICE attribution.
• Bump VERSION and CHANGELOG for v0.2.1 release prep.
• Refresh the top README with version/license/install/release metadata, direct links to individual library READMEs, Homebrew companion imports, and a license section.
• Add validation that README release metadata stays aligned with VERSION and Apache-2.0.

Issue

Closes #36

Validation

• ./tests/validate.sh
  • 135 BATS tests
  • ShellCheck
  • examples/std-usage.sh
• git diff --check
• git diff --cached --check

Notes

The Homebrew tap needs a follow-up formula update after v0.2.1 is released, because the current formula URL points at the v0.2.0 tarball with the old license.

Checklist

• Branch name follows <category>/<issue>-<YYYYMMDD>-<slug>.
• Pull request is scoped to one issue, unless a documented multi-issue exception applies.
• Pull request body explains what changed and how it was validated.
• Relevant project checks pass locally.
• Documentation is updated when behavior or user-facing commands change.
• CHANGELOG is updated for notable user-visible or release-worthy changes.
• Pull request includes Fixes #<issue> or Closes #<issue> when merge should close the issue.